Deluge removes large grub (4)
I believe the answer is:
food
'grub' is the definition.
(I've seen this before)
'deluge removes large' is the wordplay.
'deluge' becomes 'flood' (synonyms).
'removes' suggests deleting specific letters.
'large' becomes 'l'.
'flood' with 'l' removed is 'FOOD'.
(Other definitions for food that I've seen before include "Grub" , "course?" , "Nutritious substance" , "Sustenance" , "Fast --; -- chain" .)