Tar goes round and round dead ends (3,3)
I believe the answer is:
sea dog
'tar' is the definition.
(synonyms)
'goes round and round dead ends' 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 in the answer.
'dead' could be 'sea' and 'sea' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sea dog that I've seen before include "Experienced mariner" , "Old tar" , "mythical creature" , "Ancient Mariner" , "An old sailor of canine origin" .)