Imagine vines developing to maturity (8)
I believe the answer is:
envisage
'imagine' is the definition.
('envisage' can be a synonym of 'imagine')
'vines developing to maturity' is the wordplay.
'developing' indicates an anagram.
'to maturity' becomes 'age' (I've seen this before).
'vines' is an anagram of 'envis'.
'envis'+'age'='ENVISAGE'
(Other definitions for envisage that I've seen before include "Visualise - vine's age (anag)" , "Form a mental image of" , "Consider, contemplate" , "Mentally picture" , "Visualise, imagine" .)