School mentor’s No 1 reject having been accepted for United? (8)
I believe the answer is:
combined
'united?' is the definition.
(uniting is a kind of combining)
'school mentor's no 1 reject having been accepted' is the wordplay.
'school mentor' becomes 'combed' (I can't justify this - if you can you should believe this answer much more).
'no' becomes 'n' (common abbreviation).
'c' becomes 'i' ('i' can be a synonym of 'c').
'reject' indicates an anagram (I've seen 'rejecting' mean this).
'having been accepted' is an insertion indicator.
'n'+'i'='ni'
'ni' with letters rearranged gives 'in'.
'combed' going around 'in' is 'COMBINED'.
'for' is the link.
(Other definitions for combined that I've seen before include "joint solution" , "Joined together" , "Merged" .)