Encounter with Mac could become unromantic (3,4)
I believe the answer is:
run into
'encounter' is the definition.
(synonyms)
'mac could become unromantic' is the wordplay.
I cannot really see how this works, but
'become' could be 'run' (running is a kind of becoming) and 'run' is present in the answer.
The remaining letters 'into' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
'with' is the link.
(Other definitions for run into that I've seen before include "Encounter (someone)" , "Meet (someone) unexpectedly" , "charge?" , "Happen to meet - with a bump?" , "Meet (a person) by chance" .)