Painter upsetting one young man (4)
I believe the answer is:
dali
'painter' is the definition.
(I've seen this before)
'upsetting one young man' is the wordplay.
'upsetting' says the letters should be written backwards.
'one' becomes 'i' (Roman numeral).
'young man' becomes 'lad' (lad can mean a boy or young man).
'i'+'lad'='ilad'
'ilad' reversed gives 'DALI'.
(Other definitions for dali that I've seen before include "Salvador -, twentieth century artist" , "Surrealist Spanish painter, d. 1989" , "Controversial painter" , "Salvador --, twentieth century Spanish surrealist painter" , "Salvador among artists" .)