Exploit middle class (4)
I believe the answer is:
milk
'exploit' is the definition.
(to milk something is to exploit its potential)
'middle class' is the wordplay.
'middle' becomes 'M' (abbreviation for middle).
'class' becomes 'ilk' (both can mean a type or variety).
'm'+'ilk'='MILK'
(Other definitions for milk that I've seen before include "Use" , "Exploit" , "an early delivery?" , "A white drink" , "Extract money from" .)