Chief character in caper is hostile (12)
I believe the answer is:
antagonistic
'hostile' is the definition.
(similar in meaning)
'chief character in caper' is the wordplay.
'chief character' becomes 'agonist' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'caper' becomes 'antic' (synonyms).
'agonist' placed into 'antic' is 'ANTAGONISTIC'.
'is' is the link.
Can you help me to learn more?
(Other definitions for antagonistic that I've seen before include "Ill-disposed" , "Actively hostile" , "Actively opposing - stag in action (anag)" .)