Old coin, one blocking vent (5)
I believe the answer is:
ducat
'old coin' is the definition.
(I've seen this before)
'one blocking vent' is the wordplay.
I cannot quite see how this works, but
'one' could be 'a' (a thing is one thing) and 'a' is present in the answer.
The remaining letters 'duct' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for ducat that I've seen before include "Former European gold coin" , "Formerly a gold coin of several European countries" , "Ancient coin" , "piece of gold" , "Old Italian coin" .)