Hard to stop virtuoso for long (4)
I believe the answer is:
ache
'long' is the definition.
(synonyms)
'hard to stop virtuoso' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'to stop' is an insertion indicator.
'virtuoso' becomes 'ace' (synonyms).
'h' put within 'ace' is 'ACHE'.
'for' acts as a link.
(Other definitions for ache that I've seen before include "Feel pain" , "Dull prolonged pain" , "Throb - hurt" , "strong feeling" , "Play up" .)