Instrument softly accompanies regular parts in theatre (4)
I believe the answer is:
harp
'instrument' is the definition.
(I know that Harp is a instrument)
'softly accompanies regular parts in theatre' is the wordplay.
'softly' becomes 'p' (short for piano, musical term for softly).
'accompanies' is a charade indicator (letters next to each other).
'regular parts in' means one should take alternating letters.
The alternating letters of 'theatre' are 'har'.
'p' after 'har' is 'HARP'.
(Other definitions for harp that I've seen before include "Instrument (for angels?)" , "Musical symbol" , "semitone higher, abandoning intro" , "Stick to one theme" , "Traditional Gaelic musical instrument" .)