Ship's engineers rush up (6)
I believe the answer is:
trader
'ship's' is the definition.
(trading ship)
'engineers rush up' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'rush' becomes 'dart' (both can mean to move quickly).
'up' says the letters should be written backwards (in down clue: letters go upwards).
're'+'dart'='redart'
'redart' written backwards gives 'TRADER'.
(Other definitions for trader that I've seen before include "Tarred the dealer badly" , "He buys and sells goods" , "Person in the business of buying and selling" , "Tarred like merchant" , "Merchant ship; dealer" .)