Insider from Acton out to grab prize (8)
I believe the answer is:
occupant
'insider' is the definition.
(I've seen this before)
'acton out to grab prize' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'to grab' means one lot of letters goes inside another.
'prize' becomes 'cup' (cup is a kind of prize).
'acton' anagrammed gives 'ocant'.
'ocant' enclosing 'cup' is 'OCCUPANT'.
'from' acts as a link.
(Other definitions for occupant that I've seen before include "Holder of a position or office" , "Insider" , "Person residing in a property" , "Inmate" , "Indweller" .)