Old man's wise way (7)
I believe the answer is:
passage
'way' is the definition.
(I know that passage is a type of way)
'old man's wise' is the wordplay.
'old man's' becomes 'pa's' (both can mean someone's father).
'wise' becomes 'sage' (similar in meaning).
'pas'+'sage'='PASSAGE'
(Other definitions for passage that I've seen before include "Way through; part of text" , "Sea or air journey" , "street" , "Book extract; transit" , "Piece of text or music" .)