Full service in gym following Religious Education (7)
I believe the answer is:
replete
'full' is the definition.
('replete' can be a synonym of 'full')
'service in gym following religious education' is the wordplay.
'service' becomes 'let' (let is a kind of service**).
'in' indicates putting letters inside.
'gym' becomes 'pe'.
'following' means one lot of letters go next to another.
'religious education' becomes 're'.
'let' inserted into 'pe' is 'plete'.
'plete' put after 're' is 'REPLETE'.
(Other definitions for replete that I've seen before include "Gorged, satiated" , "Full, satiated" , "Full of food, gorged" , "Full - pet leer (anag)" , "Fully satisfied" .)