More pleasant French city on river (5)
I believe the answer is:
nicer
'more pleasant' is the definition.
('nice' can be similar in meaning to 'pleasant')
'french city on river' is the wordplay.
'french city' becomes 'nice' (Nice is a French city).
'on' says to put letters next to each other.
'river' becomes 'r' (abbreviation for river).
'nice'+'r'='NICER'
(Other definitions for nicer that I've seen before include "harder to please" , "better" , "relatively pleasant" , "More pleasant or courteous" , "Kinder; finer" .)