Small, deficient track (5)
I believe the answer is:
spoor
'track' is the definition.
(spoor is a kind of track)
'small deficient' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'deficient' becomes 'poor' ('poor' can be similar in meaning to 'deficient').
's'+'poor'='SPOOR'
(Other definitions for spoor that I've seen before include "Animal's track/scent" , "24 [TRAIL]" , "Animal's track or scent" , "Animal track, trail" , "Beast's trail" .)