Mother saw cook about the first Christmas birds (6)
I believe the answer is:
macaws
'birds' is the definition.
(macaw is a kind of bird)
'mother saw cook about the first christmas' is the wordplay.
'mother' becomes 'ma' (ma is a kind of mother).
'cook' indicates anagramming the letters.
'about' indicates putting letters inside.
'the first' suggests taking the first letters.
The first letter of 'christmas' is 'c'.
'saw' anagrammed gives 'aws'.
'aws' placed around 'c' is 'caws'.
'ma'+'caws'='MACAWS'
(Other definitions for macaws that I've seen before include "Long-tailed, brilliantly-coloured parrots" , "More than one bird" , "Long-tailed parrots of Central and South America" , "Birds" , "Brightly-coloured parrots" .)