Dark-coloured rook that is swallowing cold grains (5,4)
I believe the answer is:
brown rice
'grains' is the definition.
(I have seen 'Dark grains' mean 'brown rice' so perhaps 'grains' could also mean 'brown rice')
'dark-coloured rook that is swallowing cold' is the wordplay.
'dark-coloured' becomes 'brown' (brown is a dark colour).
'rook' becomes 'R' (chess abbreviation).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'swallowing' is an insertion indicator.
'cold' becomes 'c' (eg on taps).
'ie' placed around 'c' is 'ice'.
'brown'+'r'+'ice'='BROWN RICE'
(Other definitions for brown rice that I've seen before include "Health food" , "Unpolished food grains" , "Dark grains" .)