At 59, he's had only half a life! (5)
I believe the answer is:
felix
'at 59 he's had only half a' is the definition.
The definition suggests a singular noun which matches the answer.
'life' is the wordplay.
I cannot really understand how this works, but
an anagram of 'life' is 'feli' which is present in the answer.
A single letter 'x' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for felix that I've seen before include "Chap" , "M. name, literally happy" .)