Train once a boat (5)
I believe the answer is:
canoe
'boat' is the definition.
(canoeing is a kind of boating)
'train once a' is the wordplay.
'train' indicates anagramming the letters (to train can mean to grow something into a shape).
'once'+'a'='oncea'
'oncea' with letters rearranged gives 'CANOE'.
(Other definitions for canoe that I've seen before include "Kayak-like boat" , "Small and light boat, pointed at both ends" , "Boat one paddles" , "Kayak, for instance" , "River craft" .)