Most guns past their use-by date are still for sale (6)
I believe the answer is:
unsold
'use-by date are still for sale' is the definition.
I can't tell whether this definition defines the answer.
'most guns past' is the wordplay.
'most' means to remove the first letter.
'past' becomes 'old' (associated in meaning).
'guns' with its first letter taken off is 'uns'.
'uns'+'old'='UNSOLD'
'their' is the link.
Can you help me to learn more?
(Other definitions for unsold that I've seen before include "Not yet bought" , "Not convinced of merits" , "left on the shelf" , "still in the shop" , "Remaining for sale" .)