Underdone morsel of food (7)
I believe the answer is:
rarebit
'food' is the definition.
(rarebit is a kind of food)
'underdone morsel' is the wordplay.
'underdone' becomes 'rare' ('rare' can be a synonym of 'underdone').
'morsel' becomes 'bit' ('bit' can be a synonym of 'morsel').
'rare'+'bit'='RAREBIT'
'of' is the link.
(Other definitions for rarebit that I've seen before include "Cooked cheese on toast" , "mouthful?" , "Cheese/toast savoury" , "Arbiter of Welsh dish with melted cheese on toast" , "Arbiter of traditional Welsh dish" .)