Benefit from old conspiracy enveloping island (7)
I believe the answer is:
exploit
'benefit from' is the definition.
(thesaurus)
'old conspiracy enveloping island' is the wordplay.
'old' becomes 'ex'.
'conspiracy' becomes 'plot' (I've seen this before).
'enveloping' is an insertion indicator.
'island' becomes 'i'.
'plot' going around 'i' is 'ploit'.
'ex'+'ploit'='EXPLOIT'
(Other definitions for exploit that I've seen before include "Daring feat; use unfairly" , "Use to advantage" , "(Mis)use" , "Make capital out of" , "Take advantage of - feat" .)