Beginning to describe strange percussion instrument (4)
I believe the answer is:
drum
'percussion instrument' is the definition.
(drum is a kind of percussion instrument)
'beginning to describe strange' is the wordplay.
'beginning to' suggests taking the first letters.
'strange' becomes 'rum' (I've seen this before).
The initial letter of 'describe' is 'd'.
'd'+'rum'='DRUM'
(Other definitions for drum that I've seen before include "Iris rootstock (perfume)" , "Thing to be beaten - but not as a container!" , "See 18" , "See 16" , "beat" .)