Merry Parisian is diving into English river (6)
I believe the answer is:
festal
'merry' is the definition.
(thesaurus)
'parisian is diving into english river' is the wordplay.
'parisian is' becomes 'est' ('is' in French).
'diving into' means one lot of letters goes inside another.
'english river' becomes 'fal' (river in Cornwall).
'est' going within 'fal' is 'FESTAL'.
(Other definitions for festal that I've seen before include "Joyous, festive" , "Relating to a festival; joyous" , "Jovially celebratory" , "Jovial" , "Relating to a holiday" .)