A major, now - at university, a lieutenant (5)
I believe the answer is:
adult
'a major' is the definition.
(thesaurus)
'now at university a lieutenant' is the wordplay.
'now' becomes 'ad' (I've seen this in other clues).
'at' says to put letters next to each other.
'university' becomes 'u'.
'a lieutenant' becomes 'lt'.
'ad'+'u'+'lt'='ADULT'
(Other definitions for adult that I've seen before include "Person of mature age" , "one no longer in minority" , "Supposedly "responsible" person" , "not green" , "Indecent" .)