Uncle Sam, for instance, rejected convention (5)
I believe the answer is:
usage
'convention' is the definition.
(I've seen this in another clue)
'uncle sam for instance rejected' is the wordplay.
'uncle sam' becomes 'usa' (I've seen this in another clue).
'for instance' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
'rejected' says the letters should be written backwards.
'eg' backwards is 'ge'.
'usa'+'ge'='USAGE'
(Other definitions for usage that I've seen before include "Employment - practice" , "Accepted or habitual practice" , "Way of treating" , "Custom; practice" , "Habitual custom" .)