Area certain to have plenty (6)
I believe the answer is:
abound
'to have plenty' is the definition.
(to abound is to be full of something)
'area certain' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'certain' becomes 'bound' (as in the phrase 'it's bound to be').
'a'+'bound'='ABOUND'
(Other definitions for abound that I've seen before include "Be in plentiful supply" , "Be prolific" , "flourish" , "Be plentiful, say fish in river" , "Be in ample supply" .)