Racket includes old vessel (4)
I believe the answer is:
boat
'vessel' is the definition.
(I know that boat is a type of vessel)
'racket includes old' is the wordplay.
'racket' becomes 'bat' (bat is a kind of racket).
'includes' means one lot of letters goes inside another.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'bat' going around 'o' is 'BOAT'.
(Other definitions for boat that I've seen before include "Vehicle for travelling over water" , "cat, for example" , "Floating vessel" , "Small vessel" , "Contest on the Thames" .)