Ship back from Calais on purpose (4)
I believe the answer is:
send
'ship' is the definition.
(I know that ship can be written as send)
'back from calais on purpose' is the wordplay.
'back from' suggests the final letters (the letter at the back of the word).
'on' says to put letters next to each other.
'purpose' becomes 'end' (both can mean a goal).
The last letter of 'calais' is 's'.
's'+'end'='SEND'
(Other definitions for send that I've seen before include "Transport" , "Transmit (message)" , "Despatch, remit" , "Transmit; post" , "Forward" .)