Relaxed about pressure in argument (4)
I believe the answer is:
spat
'argument' is the definition.
(a spat is a disagreement or argument)
'relaxed about pressure' is the wordplay.
'relaxed' becomes 'sat' (sitting is a type of relaxing).
'about' is an insertion indicator.
'pressure' becomes 'p' (scientific abbreviation).
'sat' enclosing 'p' is 'SPAT'.
'in' acts as a link.
(Other definitions for spat that I've seen before include "Reacted contemptuously" , "Slight quarrel - ejected saliva" , "Little quarrel - leg wear" , "Spoke violently" , "Old-fashioned legwear" .)