He's nothing if not a survivor! (6)
I believe the answer is:
oliver
'he's' is the definition.
'not a survivor' is the wordplay.
'not' becomes 'o' (I've seen this in other clues).
'a survivor' becomes 'liver' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'o'+'liver'='OLIVER'
'nothing if' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for oliver that I've seen before include "Cook" , "Cromwell, for example" , "* [DICKENS WORK] Untwisted" , "First name of Goldsmith, great 18th century Irish writer" , "Dickens boy who asked for more, and a musical comedy" .)