Unfriendly, small, and petty quarrel (5)
I believe the answer is:
stiff
'unfriendly' is the definition.
(thesaurus)
'small and petty quarrel' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'and' is a charade indicator (letters next to each other).
'petty quarrel' becomes 'tiff' (I've seen this before).
's'+'tiff'='STIFF'
(Other definitions for stiff that I've seen before include "Frigid" , "Rigid, prim" , "Unyielding" , "Unfriendly" , "18 [HARD]" .)