Goes back to lop a rabbit's tail (4)
I believe the answer is:
scut
'a rabbit's tail' is the definition.
(I've seen this before)
'goes back to lop' is the wordplay.
'back' indicates one should take the final letters.
'to lop' becomes 'cut' ().
The final letter of 'goes' is 's'.
's'+'cut'='SCUT'
(Other definitions for scut that I've seen before include "Deer's tail" , "(Hare's) short upright tail" , "Cuts into a small tail" , "Small tail as of rabbit" , "Short tail of hare" .)