Sponsor a mad cardinal (11)

I believe the answer is:

fundamental

'cardinal' is the definition.
('fundamental' can be a synonym of 'cardinal')

'sponsor a mad' is the wordplay.
'sponsor' becomes 'fund' (I've seen this before).
'mad' becomes 'mental'.
'fund'+'a'+'mental'='FUNDAMENTAL'

(Other definitions for fundamental that I've seen before include "Primary" , "Of central importance" , "Basic" , "key" .)

I've seen this clue in The Times.

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