Pretend to agree with expression of doubt, filling time (6)
I believe the answer is:
humour
'pretend to agree' is the definition.
(as in humouring someone)
'expression of doubt filling time' is the wordplay.
'expression of doubt' becomes 'um' (um is said in hesitation).
'filling' indicates putting letters inside.
'time' becomes 'hour' (unit of time).
'um' put into 'hour' is 'HUMOUR'.
'with' is the link.
(Other definitions for humour that I've seen before include "See 12" , "Mood - comicality" , "gags?" , "Bodily fluid" , "Mood or something funny" .)