Supporter of United admitted to the Saints' ground (10)
I believe the answer is:
enthusiast
'supporter' is the definition.
(I know that supporter can be written as enthusiast)
'united admitted to the saints' ground' is the wordplay.
'united' becomes 'U' (as in Man U, English football team).
'admitted to' indicates putting letters inside.
'ground' indicates anagramming the letters ('grind' the letters into a new form).
'the'+'saints'='thesaints'
'thesaints' anagrammed gives 'enthsiast'.
'u' inserted into 'enthsiast' is 'ENTHUSIAST'.
'of' is the link.
(Other definitions for enthusiast that I've seen before include "buff" , "Ardent supporter" , "one not at all cool" , "Fan" , "One deeply interested in a subject" .)