Racket around old vessel (4)
I believe the answer is:
boat
'vessel' is the definition.
(boat is a kind of vessel)
'racket around old' is the wordplay.
'racket' becomes 'bat' (bat is a kind of racket).
'around' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'bat' enclosing 'o' is 'BOAT'.
(Other definitions for boat that I've seen before include "See 12 Down" , "Contest on the Thames" , "Small ship" , "Water (or gravy) vessel" , "Water transport" .)