Fellow Christian’s senior officer and better half (8)
I believe the answer is:
blighter
'fellow' is the definition.
(synonyms)
'officer and better half' is the wordplay.
'officer' becomes 'bligh' (I can't explain this - if you can you should give a lot more credence to this answer).
'and' means one lot of letters go next to another.
'better half' becomes 'ter'.
'bligh'+'ter'='BLIGHTER'
'christian's senior' is the 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 blighter that I've seen before include "Pestilential fellow" , "Wretch" , "so-and-so" , "Villain" , "irritating guy" .)