University on top of feasible merger (5)
I believe the answer is:
union
'merger' is the definition.
(I've seen this before)
'university on top of feasible' is the wordplay.
'university' becomes 'uni' (informal abbreviation).
'on top of' means one lot of letters go next to another.
'feasible' becomes 'on' (as opposed to 'not on' -unacceptable or unfeasible).
'uni'+'on'='UNION'
(Other definitions for union that I've seen before include "workers' co-operative" , "Wedding, say" , "Workers' organisation or marriage" , "Workers' group" , "Group who may want better conditions" .)