Detective sergeant collecting unusual percussion instruments (5)
I believe the answer is:
drums
'percussion instruments' is the definition.
(drum is a kind of percussion instrument)
'detective sergeant collecting unusual' is the wordplay.
'detective sergeant' becomes 'ds' (abbreviation for detective sergeant).
'collecting' is an insertion indicator.
'unusual' becomes 'rum' (I've seen this before).
'ds' enclosing 'rum' is 'DRUMS'.
(Other definitions for drums that I've seen before include "Percussion instruments" , "Containers" .)