Volunteered a bit of ‘Offside! Ref! Foul!’ but it's rebuffed (7)
I believe the answer is:
offered
'volunteered' is the definition.
('offer' can be a synonym of 'volunteer')
'a bit of offside ref foul but it's rebuffed' is the wordplay.
'a bit of' indicates a hidden word.
'but it's rebuffed' is a reversal indicator (letters sent back or rebuffed).
'dereffo' can be found hidden inside 'offside ref foul'.
'dereffo' backwards is 'OFFERED'.
(Other definitions for offered that I've seen before include "Made a bid" , "Put forward" , "Made available" , "Volunteered" , "Tendered" .)