Vigorous university student turned to pen (5)
I believe the answer is:
lusty
'vigorous' is the definition.
(I've seen this before)
'university student turned to pen' is the wordplay.
'university' becomes 'U' (abbreviation for university).
'student' becomes 'L' (as in L-plates for learner drivers).
'turned' says the letters should be written backwards.
'to pen' becomes 'sty' (both can mean an animal enclosure).
'u'+'l'='ul'
'ul' in reverse letter order is 'lu'.
'lu'+'sty'='LUSTY'
(Other definitions for lusty that I've seen before include "Full-blooded" , "Consumed with carnal desire" , "Stout" , "Strapping" , "Full of strength and energy" .)