Boat toilet starts to overflow ... eww! (5)
I believe the answer is:
canoe
'boat' is the definition.
(canoe is a kind of boat)
'toilet starts to overflow eww' is the wordplay.
'toilet' becomes 'can' ('can' can be a synonym of 'toilet').
'starts to' says to take the initial letters.
The initial letters of 'overflow eww' is 'oe'.
'can'+'oe'='CANOE'
(Other definitions for canoe that I've seen before include "Light boat propelled by paddle(s)" , "River craft" , "Simple boat" , "Boat that's paddled" , "Primitive boat" .)