Democrat party leader departing in boat (4)
I believe the answer is:
dory
'boat' is the definition.
(dory is a kind of boat)
'democrat party leader departing' is the wordplay.
'democrat' becomes 'd'.
'party' becomes 'Tory'.
'leader departing' means to remove the first letter.
'tory' with its first letter taken away is 'ory'.
'd'+'ory'='DORY'
'in' acts as a link.
(Other definitions for dory that I've seen before include "Sea-wolf" , "another fish wanted" , "Golden-yellow fish" , "Fish for a small boat" , "Fish of mackerel family" .)