Is a stand-alone magnetic compass required for an IFR fight when the aircraft has a glass panel with a Horizontal Situation Indicator?
A magnetic heading indicator is required, but not necessarily a wet compass, like this one:
Image Source: Wikipedia - Author: Chopper
Digital devices or combined devices are also possible:
Image Source: WikiPedia - Author: Mysid
The required equipment for VFR flights states, as already posted in this question:
§91.205 Powered civil aircraft with standard category U.S. airworthiness certificates: Instrument and equipment requirements.
(a) General. Except as provided in paragraphs (c)(3) and (e) of this section, no person may operate a powered civil aircraft with a standard category U.S. airworthiness certificate in any operation described in paragraphs (b) through (f) of this section unless that aircraft contains the instruments and equipment specified in those paragraphs (or FAA-approved equivalents) for that type of operation, and those instruments and items of equipment are in operable condition.
(b) Visual-flight rules (day). For VFR flight during the day, the following instruments and equipment are required:
(3) Magnetic direction indicator.
The text further states for IFR:
(d) Instrument flight rules. For IFR flight, the following instruments and equipment are required:
(1) Instruments and equipment specified in paragraph (b) of this section, and, for night flight, instruments and equipment specified in paragraph (c) of this section.
If you mean a "whiskey compass," like the thing that sits above the center of the panel, then the answer is no.
For aircraft certified under Part 23, as described in AC 23.1311-1C:
Section 23.1303(c), Amendment 23-62, amended the requirement from “A direction indicator (non-stabilized magnetic compass)” to “A magnetic direction indicator.” As new technology becomes more affordable for part 23 airplanes, many electronic flight instrument systems will use magnetically stabilized direction indicators (or electric compass systems) to measure and indicate the airplane heading to provide better performance.
A fluxgate compass, which can be remotely mounted in the wing or the tail would also satisfy the regs.