Deck is ship-shape (4)
I believe the answer is:
trim
'deck' is the definition.
(I've seen this in another clue)
'ship-shape' is the wordplay.
'ship' becomes 't' (I can't justify this - if you can you should give a lot more credence to this answer).
'shape' becomes 'rim' (rim is a kind of shape).
't'+'rim'='TRIM'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for trim that I've seen before include "Neaten by cutting" , "Edging material - in good order" , "Make smooth by cutting away irregular parts" , "Arrange sails to suit the weather" , "Spick and span" .)