Second argument is formal (5)
I believe the answer is:
stiff
'formal' is the definition.
('stiff' can be similar in meaning to 'formal')
'second argument' is the wordplay.
'second' becomes 's' (common abbreviation).
'argument' becomes 'tiff' (I've seen this before).
's'+'tiff'='STIFF'
'is' is the link.
(Other definitions for stiff that I've seen before include "cheat" , "18 [HARD]" , "Awkward" , "Rigid, unbending" , "Unyielding" .)