Old man has no use for impressive collection (7)
I believe the answer is:
panoply
'impressive collection' is the definition.
(I've seen this in another clue)
'old man has no use' is the wordplay.
'old man' becomes 'pa' (both can mean someone's father).
'has' means one lot of letters go next to another.
'use' becomes 'ply' (plying is a kind of using).
'pa'+'no'+'ply'='PANOPLY'
'for' acts as a link.
(Other definitions for panoply that I've seen before include "brilliant array" , "A complete or splendid array" , "Trappings" , "Impressive display" , "Full array, armour" .)