Crossword Genius

Head of state's mature wise man (4)

Ross

I believe the answer is:

sage

'wise man' is the definition.
(sage is a kind of wise man)

'head of state's mature' is the wordplay.
'head of' says to take the initial letters.
'mature' becomes 'age' ('age' can be a synonym of 'mature'**).
The first letter of 'state' is 's'.
's'+'age'='SAGE'

(Other definitions for sage that I've seen before include "Profoundly wise" , "Wise man - flavoursome plant" , "Man of wisdom" , "Wise woman" , "Fragrant herb having wisdom" .)

I've seen this clue in The Independent.
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