Lost war, so El Cid became a famous poet and playwright (5,5)
I believe the answer is:
oscar wilde
'playwright' is the definition.
(I've seen this before)
'lost war so el cid' is the wordplay.
'lost' indicates anagramming the letters.
'war'+'so'+'el'+'cid'='warsoelcid'
'warsoelcid' anagrammed gives 'OSCAR WILDE'.
'became a famous poet and' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for oscar wilde that I've seen before include "Irish-born writer and wit (1854-1900)" , "Irish writer and wit" , "playwright" , "He had nothing to declare but his genius" , "dramatist" .)