Old coin is article found in tube (5)
I believe the answer is:
ducat
'old coin' is the definition.
(I've seen this before)
'article found in tube' is the wordplay.
'article' becomes 'a' (the indefinite article in English).
'found in' indicates putting letters inside.
'tube' becomes 'duct' (type of tube or canal).
'a' going inside 'duct' is 'DUCAT'.
'is' is the link.
(Other definitions for ducat that I've seen before include "Money no longer" , "Historical coin" , "Old Italian coin" , "piece of gold" , "Former Eur. gold coin" .)