Cheats on copper getting caught by king and ancient saint (8)
I believe the answer is:
cuckolds
'cheats on' is the definition.
('cuckold' can be a synonym of 'cheat on')
'by king and ancient saint' is the wordplay.
'by' becomes 'cuc' (I can't explain this - if you can you should give a lot more credence to this answer).
'king' becomes 'k'.
'and' says to put letters next to each other.
'ancient' becomes 'old' ('old' can be similar in meaning to 'ancient').
'saint' becomes 's'.
'cuc'+'k'+'old'+'s'='CUCKOLDS'
'copper getting caught' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
(Other definitions for cuckolds that I've seen before include "Betrayed husbands" , "Unfaithful wives' husbands" .)