Radiantly happy, remained drinking whiskey by river (6-4)
I believe the answer is:
starry-eyed
'radiantly happy' is the definition.
'remained drinking whiskey by river' is the wordplay.
'remained' becomes 'stayed' (staying is a kind of remaining).
'drinking' is an insertion indicator (inserted letters are consumed or drunk).
'whiskey' becomes 'rye whiskey' (rye whiskey is a type of whiskey).
'by' means one lot of letters go next to another.
'river' becomes 'r' (abbreviation).
'rye' put after 'r' is 'rrye'.
'stayed' going around 'rrye' is 'STARRY-EYED'.
(Other definitions for starry-eyed that I've seen before include "Romantically optimistic" , "Naively idealistic" , "Out of touch with reality" , "Naively enthusiastic" , "Full of unsophisticated optimism" .)