A structural member at right angles to a ship (5)
I believe the answer is:
abeam
'at right angles to a ship' is the definition.
(I've seen this in another clue)
'a structural member' is the wordplay.
'structural member' becomes 'beam' (beam is a kind of structural member).
'a'+'beam'='ABEAM'
(Other definitions for abeam that I've seen before include "at right angles to vessel" , "across width of ship" , "going from one side to the other" , "across centre" , "from side to side on voyage?" .)