Upset Argyle lost — not entirely just! (6)
I believe the answer is:
solely
'just' is the definition.
(solely means just or only)
'upset argyle lost not entirely' is the wordplay.
'upset' says the letters should be written in reverse.
'not entirely' indicates the answer is hidden within the clue.
'ylelos' is hidden within 'argyle lost'.
'ylelos' backwards is 'SOLELY'.
(Other definitions for solely that I've seen before include "nowhere else?" , "Exclusively, only" , "Singly" , "alone" , "Singularly" .)