Heroic nobleman imprisoned in iron ship (8)
I believe the answer is:
fearless
'heroic' is the definition.
(thesaurus)
'nobleman imprisoned in iron ship' is the wordplay.
'nobleman' becomes 'earl' (member of the nobility).
'imprisoned in' is an insertion indicator.
'iron' becomes 'Fe' (chemical symbol for iron).
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'earl' put inside 'fe' is 'fearle'.
'fearle'+'ss'='FEARLESS'
(Other definitions for fearless that I've seen before include "Bold" , "Intrepid" , "game?" , "Oblivious of dangers or resolute in facing them" , "Hardy" .)