Instrument behind keeping animal doctor (4,4)
I believe the answer is:
bass drum
'instrument' is the definition.
(bass drum is a kind of instrument)
'behind keeping animal doctor' is the wordplay.
'behind' becomes 'bum' ('bum' can be a synonym of 'behind'**).
'keeping' is an insertion indicator.
'animal' becomes 'ass' (I've seen this before).
'doctor' becomes 'dr' ('Dr' can be a synonym of 'doctor').
'ass'+'dr'='assdr'
'bum' placed around 'assdr' is 'BASS DRUM'.
(Other definitions for bass drum that I've seen before include "Big percussion instrument" , "Deep instrument" , "Large percussion instrument" , "musical instrument" , "Booming percussion instrument" .)