Old money initially accepted in Tube (5)
I believe the answer is:
ducat
'old money' is the definition.
(historical type of coin)
'initially accepted in tube' is the wordplay.
'initially' suggests taking the first letters.
'in' indicates putting letters inside.
'tube' becomes 'duct' (type of tube or canal).
The initial letter of 'accepted' is 'a'.
'a' inserted inside 'duct' is 'DUCAT'.
(Other definitions for ducat that I've seen before include "piece of gold" , "Old Italian coin" , "Antique coin" , "Mad; fruit" , "ready no longer" .)