Learner, university girl, entertaining trendy celebrity (8)
I believe the answer is:
luminary
'celebrity' is the definition.
(luminary is a kind of celebrity)
'learner university girl entertaining trendy' is the wordplay.
'learner' becomes 'l'.
'university' becomes 'u'.
'girl' becomes 'mary' (**).
'entertaining' means one lot of letters goes inside another.
'trendy' becomes 'in'.
'mary' enclosing 'in' is 'minary'.
'l'+'u'+'minary'='LUMINARY'
(Other definitions for luminary that I've seen before include "Person who inspired others" , "Eminent person or light-giving body" , "Leading light" , "One famous in his profession" , "famous person" .)