Small argument, formal (5)
I believe the answer is:
stiff
'formal' is the definition.
(similar in meaning)
'small argument' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'argument' becomes 'tiff' (I've seen this before).
's'+'tiff'='STIFF'
(Other definitions for stiff that I've seen before include "Formal" , "Difficult" , "Firm" , "Frigid" , "Starched, rigid" .)