Small argument is 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 in other clues).
's'+'tiff'='STIFF'
'is' is the link.
(Other definitions for stiff that I've seen before include "18 [HARD]" , "Severe - conventional - dead body" , "Awkward" , "Not giving" , "Firm" .)