Small argument, being unyielding (5)
I believe the answer is:
stiff
'unyielding' is the definition.
(I've seen this before)
'small argument' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'argument' becomes 'tiff' (I've seen this in another clue).
's'+'tiff'='STIFF'
'being' is the link.
(Other definitions for stiff that I've seen before include "Firm" , "Inflexible" , "One dead" , "Formal" , "Awkward" .)