Not so old, one guy adrift on river (7)
I believe the answer is:
younger
'not so old' is the definition.
('young' can be the opposite of 'old' so perhaps 'younger' is reasonable for 'not so old')
'one guy adrift on river' is the wordplay.
'adrift' is an anagram indicator.
'on' is a charade indicator (letters next to each other).
'river' becomes 'r'.
'one'+'guy'='oneguy'
'oneguy' anagrammed gives 'younge'.
'younge'+'r'='YOUNGER'
(Other definitions for younger that I've seen before include "more boyish" , "Less experienced" , "More youthful" , "Less grey" , "Less aged" .)