French resort's temperature is most agreeable (6)
I believe the answer is:
nicest
'most agreeable' is the definition.
(I've seen this before)
'french resort's temperature' is the wordplay.
'french resort's' becomes 'Nice's' (French coastal resort).
'temperature' becomes 't' (abbreviation).
'nices'+'t'='NICEST'
'is' acts as a link.
(Other definitions for nicest that I've seen before include "Sin etc (anag)" , "I scent in the most pleasant way" , "Most appealing" , "Insect (anag)" , "most appetising" .)