A large number start to retch at the back of the boat (4)
I believe the answer is:
raft
'a large number' is the definition.
(I've seen this in another clue)
'start to retch at the back of the boat' is the wordplay.
'start to' suggests taking the first letters.
'at the back of the boat' becomes 'aft' (I've seen this in another clue).
The initial letter of 'retch' is 'r'.
'r'+'aft'='RAFT'
(Other definitions for raft that I've seen before include "Simple floating structure" , "Flat floating structure used as boat" , "Primitive boat" , "Flat water craft; great collection" , "large collection" .)