Firm joins US university engaging a thousand in effect (6)
I believe the answer is:
commit
'effect' is the definition.
(thesaurus)
'firm joins us university engaging a thousand' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'joins' is a charade indicator (letters next to each other).
'us university' becomes 'mit' (I've seen this in other clues).
'engaging' is an insertion indicator.
'a thousand' becomes 'm' ('M' can be a synonym of 'thousand').
'mit' going around 'm' is 'mmit'.
'co'+'mmit'='COMMIT'
'in' acts as a link.
(Other definitions for commit that I've seen before include "Do or perform, say a crime" , "Confine" , "Entrust; carry out" , "Perpetrate - promise - put away" , "Entrust; perpetrate" .)