On reflection, nothing but one red cake (8)
I believe the answer is:
tortilla
'cake' is the definition.
(tortilla is a kind of cake)
'on reflection nothing but one red' is the wordplay.
'on reflection' says the letters should be written backwards.
'nothing but' becomes 'all' (I've seen this in other clues).
'one' becomes 'i' (Roman numeral).
'red' becomes 'trot' (Trotskyite, type of socialist or 'red').
'all'+'i'+'trot'='allitrot'
'allitrot' back-to-front is 'TORTILLA'.
(Other definitions for tortilla that I've seen before include "hot meal?" , "Thin Mexican corn cake" , "Mexican snack" , "Littoral (anag.)" , "Mexican maize pancake" .)