Nominally, he has only half a life at 59! (5)

I believe the answer is:

felix

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'nominally he' is the definition.
The definition suggests a singular noun which matches the answer.

'only half a life at 59' is the wordplay.
'only half' suggests halving the letters.
'at' is a charade indicator (letters next to each other).
'59' becomes 'LIX' (roman numeral).
'life' halved is 'fe'.
'fe'+'lix'='FELIX'

'has' acts as a link.

Can you help me to learn more?

(Other definitions for felix that I've seen before include "Chap" , "M. name, literally happy" .)

I've seen this clue in the Evening Standard.

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