Glimpse a drum kit, half missing (6)
I believe the answer is:
apercu
'glimpse' is the definition.
(I've seen this in another clue)
'a drum kit half missing' is the wordplay.
'drum kit' becomes 'percussion' (percussion instruments).
'half missing' suggests halving the letters.
'percussion' halved is 'percu'.
'a'+'percu'='APERCU'
(Other definitions for apercu that I've seen before include "summary, insight; race up (anag.)" , "9 [WHATS THE POINT]" , "brief explanation" , "Precis, brief outline" , "Brief sketch, outline" .)