Find a son determined (9)

I believe the answer is:

ascertain

'find' is the definition.
(I know that find can be written as ascertain)

'a son determined' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'determined' becomes 'certain' (certain can mean determined or decided).
'a'+'s'+'certain'='ASCERTAIN'

(Other definitions for ascertain that I've seen before include "Learn" , "Establish, find out" , "Find out - make sure" , "Cartesian (anag)" , "Verify" .)

I've seen this clue in The Guardian.

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