University can back assembly (4)
I believe the answer is:
unit
'assembly' is the definition.
(a group of people)
'university can back' is the wordplay.
'university' becomes 'u' (abbreviation for university).
'can' becomes 'tin' (synonyms).
'back' is a reversal indicator.
'tin' back-to-front is 'nit'.
'u'+'nit'='UNIT'
(Other definitions for unit that I've seen before include "A considerable number of soldiers" , "Building block - small military force" , "Part of a kit" , "Element or part" , "One; military group" .)