Old piece of key found in Tube (5)
I believe the answer is:
ducat
'old piece' is the definition.
'ducat' can be an answer for 'piece' (I have seen 'piece of gold' mean 'ducat' so perhaps 'piece' could also mean 'ducat'). I'm not sure about the 'old' bit.
'key found in tube' is the wordplay.
'key' becomes 'A' (musical key).
'found in' indicates putting letters inside.
'tube' becomes 'duct' (type of tube or canal).
'a' inserted inside 'duct' is 'DUCAT'.
'of' is the link.
Can you help me to learn more?
(Other definitions for ducat that I've seen before include "tender once" , "Cut Da out an old coin" , "Former Eur. gold coin" , "Coin once used" , "Mad; fruit" .)