Old story about one adventure (7)
I believe the answer is:
exploit
'adventure' is the definition.
(I've seen this in another clue)
'old story about one' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'story' becomes 'plot' (plot is a kind of story).
'about' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'plot' going around 'i' is 'ploit'.
'ex'+'ploit'='EXPLOIT'
(Other definitions for exploit that I've seen before include "put to good use" , "Make use of (a resource)" , "Benefit from" , "Take advantage of - feat" , "Heroic deed" .)