See a ship become a write-off (4)
I believe the answer is:
loss
'a write-off' is the definition.
(financial loss)
'see a ship' is the wordplay.
'see' becomes 'lo' ('lo' is an archaic exclamation meaning 'look!').
'a ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'lo'+'ss'='LOSS'
'become' acts as a link.
(Other definitions for loss that I've seen before include "Defeat; bereavement" , "Defeat - deprivation" , "Red figures on a balance sheet or ledger" , "Deficit" , "Financial reverse" .)