Small incision in tail (4)
I believe the answer is:
scut
'tail' is the definition.
(I know that scut is a type of tail)
'small incision' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'incision' becomes 'cut' (I've seen this before).
's'+'cut'='SCUT'
'in' acts as a link.
(Other definitions for scut that I've seen before include "Cuts into a small tail" , "Short erect tail" , "(Hare's) short upright tail" , "Short tail of hare" , "Small tail as of rabbit" .)