Group of players defending centre for silliest foul (7)
I believe the answer is:
squalid
'foul' is the definition.
(both can mean disgusting)
'group of players defending centre for silliest' is the wordplay.
'group of players' becomes 'squad' (I've seen this before).
'defending' means one lot of letters goes inside another.
'centre for' says to take the centre.
The middle of 'silliest' is 'li'.
'squad' enclosing 'li' is 'SQUALID'.
(Other definitions for squalid that I've seen before include "Extremely dirty" , "Dirty and repulsive" , "run down" , "Dirty and neglected" , "Run-down - filthy" .)