Cassette in wagon on crest (9)
I believe the answer is:
cartridge
'cassette' is the definition.
(I've seen this before)
'wagon on crest' is the wordplay.
'wagon' becomes 'cart' (cart is a kind of wagon).
'on' is a charade indicator (letters next to each other).
'crest' becomes 'ridge' (I've seen this before).
'cart'+'ridge'='CARTRIDGE'
'in' is the link.
(Other definitions for cartridge that I've seen before include "Bullet case" , "Cylindrical casing of ammunition" , "Bullet's casing" , "Shotgun ammunition" , "Tube with missile for shotgun" .)