Ruffian upset small lads (4)
I believe the answer is:
boys
'lads' is the definition.
(I've seen this in another clue)
'ruffian upset small' is the wordplay.
'ruffian' becomes 'yob' (synonyms).
'upset' shows that the letters should be reversed in order.
'small' becomes 's' (abbreviation - e.g. clothes size).
'yob' backwards is 'boy'.
'boy'+'s'='BOYS'
(Other definitions for boys that I've seen before include "Lads" , "They will be themselves, it's said" , "Male children" , "Young males" , "Young men" .)