Going off, then, to refit a vessel (2,3,4)
I believe the answer is:
on the turn
'going off' is the definition.
(I've seen this before)
'then to refit a vessel' is the wordplay.
'refit' indicates an anagram (I've seen 'refitting' mean this).
'a vessel' becomes 'urn' (urn is a kind of vessel).
'then'+'to'='thento'
'thento' anagrammed gives 'onthet'.
'onthet'+'urn'='ON THE TURN'
(Other definitions for on the turn that I've seen before include "change" , "Going sour" .)