Take advantage of an old flame - one in conspiracy! (7)
I believe the answer is:
exploit
'take advantage of' is the definition.
('exploit' can be a synonym of 'take advantage of')
'an old flame one in conspiracy' is the wordplay.
'an old flame' becomes 'ex' (I've seen this in other clues).
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'conspiracy' becomes 'plot' (I've seen this before).
'i' inserted inside 'plot' is 'ploit'.
'ex'+'ploit'='EXPLOIT'
(Other definitions for exploit that I've seen before include "Stunt" , "Heroic deed" , "Make good or selfish use of" , "Take advantage of - feat" , "Valiant deed; take advantage of" .)