It's expected they'll have an affair, naturally (1,6,2,6)
I believe the answer is:
a matter of course
'it's expected they'll' is the definition.
I can't tell whether this defines the answer.
'an affair naturally' is the wordplay.
'an' becomes 'a'.
'affair' becomes 'matter' (synonyms).
'naturally' becomes 'of course' (synonyms).
'a'+'matter'+'ofcourse'='A MATTER OF COURSE'
'have' acts as a link.
Can you help me to learn more?
(Other definitions for a matter of course that I've seen before include "Someone covering Aintree" , "routine" .)