A timber at right angles to a ship's keel (5)
I believe the answer is:
abeam
'at right angles to a ship's keel' is the definition.
(I've seen this in another clue)
'a timber' is the wordplay.
'timber' becomes 'beam' (I have seen 'Sturdy timber ' mean 'beam' so perhaps 'timber' could also mean 'beam').
'a'+'beam'='ABEAM'
(Other definitions for abeam that I've seen before include "across the deck" , "angled across the plane" , "Set across" , "In a line at right angles to a vessel's length" , "at right angles to the plane" .)