Starter of mutton to devour? (4)
I believe the answer is:
meat
'starter of mutton' is the definition.
'meat' can be an answer for 'mutton' (I've seen this in another clue). I am unsure of the 'starter of' bit.
'to devour?' is the wordplay.
'to' becomes 'm' (I can't justify this - if you can you should believe this answer much more).
'devour?' becomes 'eat' (devouring is a kind of eating).
'm'+'eat'='MEAT'
(Other definitions for meat that I've seen before include "17 [ESSENCE]" , "The flesh of animals as food" , "Animal flesh as food" , "Eg, beef or pork" , "Flesh for eating" .)