You once learnt he 'entertains' nurses (4)
I believe the answer is:
thee
'you once' is the definition.
(historical second person pronoun)
'learnt he entertains nurses' is the wordplay.
'nurses' indicates a hidden word (nurse can mean holding closely).
'THEE' can be found hidden inside 'learnt he entertains'.
(Other definitions for thee that I've seen before include "You in the olden days" , "'U'" , "Poetically you" , "Archaic form of 'you'" , "You (arch.)" .)