Unexpected consequence of backing those kissing boyfriend? (9)
I believe the answer is:
spillover
'unexpected consequence' is the definition.
(I've seen this before)
'backing those kissing boyfriend?' is the wordplay.
I cannot quite see how this works, but
'boyfriend?' could be 'lover' (boyfriend is a kind of lover) and 'lover' is located in the answer.
This explanation may well be incorrect...
'of' is the link.
Can you help me to learn more?
(Other definitions for spillover that I've seen before include "could be too much to cope with" , "excess that's spread around" , "unexpected consequence" .)