Launch sailing vessel and celebrate lavishly (4,3,4,3)
I believe the answer is:
push the boat out
I believe this clue is a double definition.
'launch sailing vessel' is the first definition.
'celebrate lavishly' is the second definition.
(I've seen this before)
'and' is the link.
(Other definitions for push the boat out that I've seen before include "Entertain or celebrate lavishly" , "get ready for a row?" , "Punters may" , "party?" , "launch" .)