Underdone piece of snack food (7)
I believe the answer is:
rarebit
'snack food' is the definition.
'rarebit' can be an answer for 'food' (rarebit is a kind of food). I'm not sure about the 'snack' bit.
'underdone piece' is the wordplay.
'underdone' becomes 'rare' (synonyms).
'piece' becomes 'bit' (synonyms).
'rare'+'bit'='RAREBIT'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for rarebit that I've seen before include "Welsh food?" , "Arbiter of tasty Welsh dish of cheese on toast" , "Savoury dish of cheese on toast" , "which the Welsh take down?" , "Cooked cheese on toast" .)