Old Roman god guards unknown ship for hero (8)
I believe the answer is:
odysseus
'for hero' is the definition.
(I've seen this before)
'old roman god guards unknown ship' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'roman god' becomes 'deus' ('god' in Latin).
'guards' is an insertion indicator (inserted letters are guarded).
'unknown' becomes 'y' (algebra).
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'y'+'ss'='yss'
'deus' enclosing 'yss' is 'dysseus'.
'o'+'dysseus'='ODYSSEUS'
(Other definitions for odysseus that I've seen before include "King of Ithaca, who returned after the siege of Troy" , "Homeric wanderer" , "Homer's mythical Greek hero" , "Classical traveller" , "Voyager" .)