Youngster and old man returning in drink (7)
I believe the answer is:
sapling
'youngster' is the definition.
(thesaurus)
'old man returning in drink' is the wordplay.
'old man' becomes 'pa' (both can mean someone's father).
'returning' says the letters should be written backwards.
'in' means one lot of letters goes inside another.
'drink' becomes 'sling' (sling is a kind of drink).
'pa' backwards is 'ap'.
'ap' placed inside 'sling' is 'SAPLING'.
'and' is the link.
(Other definitions for sapling that I've seen before include "small bit of wood" , "Little tree" , "Lapsing into a young tree" , "Young slender tree" , "Baby tree" .)