Keen little twerp's back (4)
I believe the answer is:
weep
'keen' is the definition.
(thesaurus)
'little twerp's back' is the wordplay.
'little' becomes 'wee' ('wee' can be similar in meaning to 'little').
'back' indicates one should take the final letters.
The last letter of 'twerp' is 'p'.
'wee'+'p'='WEEP'
(Other definitions for weep that I've seen before include "Keen" , "Bawl" , "Greet" , "pipe" , "Show emotion" .)