Rabbit's tail, small and trim (4)
I believe the answer is:
scut
'rabbit's tail' is the definition.
(I've seen this before)
'small and trim' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'and' means one lot of letters go next to another.
'trim' becomes 'cut' ('cut' can be a synonym of 'trim'**).
's'+'cut'='SCUT'
(Other definitions for scut that I've seen before include "(Hare's) short upright tail" , "Reindeer's tail" , "Deer's tail" , "Short tail of rabbit, deer etc" , "Short erect tail" .)