Captain eats first authentic breakfast (6)
I believe the answer is:
cereal
'breakfast' is the definition.
(I've seen this before)
'captain eats first authentic' is the wordplay.
'first' suggests taking the first letters.
'authentic' becomes 'real' (I've seen this before).
The first letters of 'captain eats' is 'ce'.
'ce'+'real'='CEREAL'
(Other definitions for cereal that I've seen before include "11 food, maybe" , "Grass producing an edible grain" , "Something for brekkie" , "Common part of breakfast" , "Edible grain for breakfast" .)