An officer or one who has taken a prisoner (6)
I believe the answer is:
captor
'prisoner' is the definition.
(I've seen this before)
'an officer or' is the wordplay.
'an officer' becomes 'capt' ().
'capt'+'or'='CAPTOR'
'one who has taken a' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for captor that I've seen before include "Someone taking prisoner" , "Holder of a prisoner" , "kidnapper, perhaps" , "Person who takes and holds" , "Guard" .)