After first half of sandwich, eats fish (8)
I believe the answer is:
sardines
'fish' is the definition.
(I've seen this before)
'after first half of sandwich eats' is the wordplay.
'after first half of sandwich' becomes 'sar' (I can't explain this - if you can you should give a lot more credence to this answer).
'eats' becomes 'dines' (dining is a kind of eating).
'sar'+'dines'='SARDINES'
Can you help me to learn more?
(Other definitions for sardines that I've seen before include "Squash-together party game" , "Young pilchards" , "Cumulative hiding game" , "Children's game - fish" , "Small fish, usually canned" .)