Cultivate a habit in American English (3)
I believe the answer is:
use
'cultivate a habit' is the definition.
(as in using drugs and getting a drug habit)
'american english' is the wordplay.
'american' becomes 'us' (abbreviation for United States).
'english' becomes 'e' (abbreviation).
'us'+'e'='USE'
'in' acts as a link.
(Other definitions for use that I've seen before include "Application" , "Profit" , "Put into service, do something with" , "Milk" , "Expend in amusement" .)