Old coin and key dropped in tube (5)
I believe the answer is:
ducat
'old coin' is the definition.
(I've seen this before)
'key dropped in tube' is the wordplay.
'key' becomes 'a' (I can't justify this - if you can you should give a lot more credence to this answer).
'dropped in' indicates putting letters inside.
'tube' becomes 'duct' (I've seen this before).
'a' inserted inside 'duct' is 'DUCAT'.
'and' is the link.
(Other definitions for ducat that I've seen before include "Old European gold or silver coin" , "Former Italian gold coin" , "Money no longer" , "Coin once used" , "Old Italian coin" .)