I have grub, regularly entering red channel (5,3)
I believe the answer is:
river bed
'channel' is the definition.
(I've seen this before)
'i have grub regularly entering red' is the wordplay.
'i have' becomes 'ive'.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'entering' means one lot of letters goes inside another.
The alternating letters of 'grub' are 'rb'.
'ive'+'rb'='iverb'
'iverb' placed inside 'red' is 'RIVER-BED'.
(Other definitions for river bed that I've seen before include "Base of natural watercourse" , "channel" .)