Flier going round old vessel (4)
I believe the answer is:
boat
'vessel' is the definition.
(I know that vessel can be written as boat)
'flier going round old' is the wordplay.
I cannot quite see how this works, but
'round' could be 'o' (round can mean a round object) and 'o' is found within the answer.
The remaining letters 'bat' 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 boat that I've seen before include "See 18" , "Vehicle for travelling over water" , "Water craft" , "See 12 Down" , "Vessel for travelling on water" .)