Argued shiftily about pressure to improve (7)
I believe the answer is:
upgrade
'improve' is the definition.
(thesaurus)
'argued shiftily about pressure' is the wordplay.
'shiftily' indicates an anagram (I've seen this in other clues).
'about' means one lot of letters goes inside another.
'pressure' becomes 'p' (abbreviation).
'argued' with letters rearranged gives 'ugrade'.
'ugrade' going around 'p' is 'UPGRADE'.
'to' is the link.
(Other definitions for upgrade that I've seen before include "Raise in status or quality" , "Get better software" , "Higher-standard version" , "Bonus for plane passenger" , "Improve, enhance" .)