Big head - a blessing for a monkey (6)

I believe the answer is:

baboon

'for a monkey' is the definition.
(baboon is a kind of monkey)

'big head a blessing' is the wordplay.
'head' indicates taking the first letters.
'blessing' becomes 'boon' (synonyms).
The first letter of 'big' is 'b'.
'b'+'a'+'boon'='BABOON'

(Other definitions for baboon that I've seen before include "Large primate" , "Type of large monkey" , "Large long-nosed African or Asian monkey" , "Uncouth type" , "Ground-living primate" .)

I've seen this clue in the Sydney Morning Herald.

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