Exploit mother-in-law, Karen, for starters (4)
I believe the answer is:
milk
'exploit' is the definition.
(to milk something is to exploit its potential)
'mother-in-law karen for starters' is the wordplay.
'for starters' indicates taking the first letters.
The initial letters of 'mother in law karen' is 'MILK'.
(Other definitions for milk that I've seen before include "an early delivery?" , "A white drink" , "Exploit" , "Source of cream/butter" , "Use" .)