A smile across the length of ship (5)
I believe the answer is:
abeam
'across the length of ship' is the definition.
(abeam can mean across a ship)
'a smile' is the wordplay.
'smile' becomes 'beam' (to beam is to smile).
'a'+'beam'='ABEAM'
(Other definitions for abeam that I've seen before include "Position relative to ship" , "at right angles to the plane" , "neither fore nor aft?" , "at right angles to vessel" , "At right angles to ship's length" .)