Pen old university student put in jacket (7)
I believe the answer is:
cagoule
'jacket' is the definition.
(cagoule is a kind of jacket)
'pen old university student put in' is the wordplay.
'pen' becomes 'cage' (I've seen this before).
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'university' becomes 'U' (abbreviation for university).
'student' becomes 'L' (as in L-plates for learner drivers).
'put in' is an insertion indicator.
'o'+'u'+'l'='oul'
'cage' placed around 'oul' is 'CAGOULE'.
(Other definitions for cagoule that I've seen before include "Hooded waterproof anarak" , "coat" , "Waterproof garment" , "Hiker's waterproof" , "Hooded jacket" .)