Groan about musical instrument (5)
I believe the answer is:
organ
'musical instrument' is the definition.
(organ is a kind of musical instrument)
'groan about' is the wordplay.
'about' indicates anagramming the letters.
'groan' is an anagram of 'ORGAN'.
(Other definitions for organ that I've seen before include "means for communicating information" , "Pipe or reed instrument" , "stops on one" , "Functional unit of the body and musical instrument" , "Musical instrument - pancreas, perhaps" .)