Substitute's deceit swallowed by football official (6)
I believe the answer is:
relief
'substitute's' is the definition.
('relief' can be a synonym of 'substitute')
'deceit swallowed by football official' is the wordplay.
'deceit' becomes 'lie' (I've seen this in other clues).
'swallowed by' is an insertion indicator.
'football official' becomes 'ref' (ref is a kind of football official).
'lie' going into 'ref' is 'RELIEF'.
(Other definitions for relief that I've seen before include "Alleviation; (financial) assistance" , "Alleviation, easing" , "new person on duty" , "replacement staff member" , "Easement of pressure or pain" .)