Artist with long hair, beatnik at heart (5)
I believe the answer is:
manet
'artist' is the definition.
(I've seen this before)
'with long hair beatnik at heart' is the wordplay.
'with' is a charade indicator (letters next to each other).
'long hair' becomes 'mane' (I've seen this before).
'at heart' indicates the central letters.
The central letter of 'beatnik' is 't'.
'mane' next to 't' is 'MANET'.
(Other definitions for manet that I've seen before include "Edouard - - (painter)" , "Olympia painter" , "Edouard ___, painted Dejeuner sur l'herbe, d. 1883" , "Olympia artist" , "Edouard -- (artist)" .)