Groan about instrument (5)
I believe the answer is:
organ
'instrument' is the definition.
(I know that organ is a type of instrument)
'groan about' is the wordplay.
'about' indicates an anagram.
'groan' anagrammed gives 'ORGAN'.
(Other definitions for organ that I've seen before include "Functional part of body" , "Instrument - heart, for example" , "Church instrument - heart, perhaps" , "Stopped instrument" , "ear maybe" .)