Mark tucking into eggs, initially came to one with shell (8)
I believe the answer is:
escargot
'one with shell' is the definition.
'mark tucking into eggs initially came' is the wordplay.
'mark' becomes 'scar' ('scar' can be a synonym of 'mark').
'tucking into' is an insertion indicator.
'initially' suggests taking the first letters.
'came' becomes 'got' ('get' can be a synonym of 'come').
The first letter of 'eggs' is 'e'.
'e'+'got'='egot'
'scar' placed into 'egot' is 'ESCARGOT'.
'to' is the link.
(Other definitions for escargot that I've seen before include "An edible snail from France" , "Cart goes carrying edible French snail" , "Got scare from edible French snail" , "One continually moving house" , "Hardly fast food" .)