Offend stranger playing with small son (10)
I believe the answer is:
transgress
'offend' is the definition.
(thesaurus)
'stranger playing with small son' is the wordplay.
'playing with' indicates anagramming the letters.
'small' becomes 's' (abbreviation - e.g. clothes size).
'son' becomes 's'.
'stranger' anagrammed with 's' makes 'transgres'.
'transgres'+'s'='TRANSGRESS'
(Other definitions for transgress that I've seen before include "misbehave" , "Cause offence" , "Break (limit, law)" , "Commit a sin" , "Overstep, exceed" .)