Brave nobleman's aboard iron ship (8)
I believe the answer is:
fearless
'brave' is the definition.
('fearless' can be a synonym of 'brave')
'nobleman's aboard iron ship' is the wordplay.
'nobleman' becomes 'earl' (earl is a kind of nobleman).
'aboard' indicates putting letters inside.
'iron' becomes 'fe' (Fe is the chemical symbol for iron).
'ship' becomes 'ss'.
'earl' going inside 'fe' is 'fearle'.
'fearle'+'ss'='FEARLESS'
(Other definitions for fearless that I've seen before include "Unafraid" , "Intrepid" , "Stout-hearted" , "Oblivious of dangers or resolute in facing them" , "Audacious" .)