Remarkably united group of students with posh friend (9)
I believe the answer is:
unusually
'remarkably' is the definition.
(synonyms)
'united group of students with posh friend' is the wordplay.
'united' becomes 'u'.
'group of students' becomes 'NUS' (National Union of Students).
'with' is a charade indicator (letters next to each other).
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'friend' becomes 'ally' (synonyms).
'u'+'nus'+'u'+'ally'='UNUSUALLY'
(Other definitions for unusually that I've seen before include "Oddly" , "Rarely" , "Not in the ordinary way" , "Remarkably, not normally" , "surprisingly" .)