Prefix with red (5)
I believe the answer is:
infra
'prefix with red' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for infra that I've seen before include "Dig is demeaning" , "Coming below" , "appearing later in book?" , "Latin adverb meaning below or further on in book" , "Beneath (esp. 15)" .)