Couple send up one who's unfaithful (3-5)
I believe the answer is:
two-timer
'one who's unfaithful' is the definition.
I can't tell whether this definition defines the answer.
'couple send up' is the wordplay.
'couple' becomes 'two' (I've seen this before).
'send' becomes 'remit' (I've seen this before**).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'remit' back-to-front is 'timer'.
'two'+'timer'='TWO-TIMER'
Can you help me to learn more?
(Other definitions for two-timer that I've seen before include "Unfaithful lover in arithmetic mode" , "Double-crosser" , "Unfaithful partner" , "One who is unfaithful to wife - twice?" , "Unfaithful person" .)