Upset by grasping old ruffian (3)
I believe the answer is:
yob
'ruffian' is the definition.
(synonyms)
'upset by grasping old' is the wordplay.
'upset' says the letters should be written backwards.
'grasping' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'by' enclosing 'o' is 'boy'.
'boy' reversed gives 'YOB'.
(Other definitions for yob that I've seen before include "Rude aggressive youth" , "Loutish young man" , "Troublemaker" , "Thug" , "Hooligan kind of boy" .)