Weakling exploits small operation (7)
I believe the answer is:
milksop
'weakling' is the definition.
(I've seen this before)
'exploits small operation' is the wordplay.
'exploits' becomes 'milks' (milking is a kind of exploiting).
'small operation' becomes 'op' ('small' asks for an abbreviation).
'milks'+'op'='MILKSOP'
(Other definitions for milksop that I've seen before include "Unassertive male person" , "Feeble and effete person" , "Weak or effeminate man" , "Inefectual man" , "Chinless wonder" .)