Poem unknown about Yankee ship's epic journey (7)
I believe the answer is:
odyssey
'epic journey' is the definition.
(journey taken by Odysseus)
'poem unknown about yankee ship's' is the wordplay.
'poem' becomes 'ode' (ode is a kind of poem).
'unknown' becomes 'y' (algebra).
'about' indicates putting letters inside.
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'ode'+'y'='odey'
'y'+'ss'='yss'
'odey' enclosing 'yss' is 'ODYSSEY'.
(Other definitions for odyssey that I've seen before include "Homer's epic poen about great journey" , "Long, eventful, wandering journey" , "Greek epic on the wanderings of Ulysses" , "Gruelling journey as by Homer" , "Homerian epic" .)