Mostly the red variety? (4)
I believe the answer is:
deer
'variety?' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'mostly the red' is the wordplay.
'mostly' becomes 'dee' (I can't explain this - if you can you should give a lot more credence to this answer).
'the red' becomes 'r' (I've seen this in other clues).
'dee'+'r'='DEER'
Can you help me to learn more?
(Other definitions for deer that I've seen before include "Perhaps fallow" , "Ruminant mammal with antlers and hooves" , "See 23" , "Like a power switch" , "The male is a stag" .)