Regulating pair first who are being unfaithful (3-6)
I believe the answer is:
two-timing
'are being unfaithful' is the definition.
('two-timing' can be similar in meaning to 'unfaithful')
'regulating pair first' is the subsidiary indication.
'regulating' becomes 'timing' (timing is a kind of regulating).
'pair' becomes 'two' (I've seen this before).
'first' means one lot of letters go next to another.
'timing' after 'two' is 'twotiming'.
'who' is the link.
(Other definitions for two-timing that I've seen before include "Double-crossing" , "Cheating on a partner" , "Being unfaithful to" , "Double-dealing" .)