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