Key language for gastropod (8)
I believe the answer is:
escargot
'gastropod' is the definition.
(escargot is an edible snail)
'key language' is the wordplay.
'key' becomes 'esc' ('Escape' key on a computer keyboard).
'language' becomes 'argot' (argot is a kind of language).
'esc'+'argot'='ESCARGOT'
'for' is the link.
(Other definitions for escargot that I've seen before include "One continually moving house" , "Cart goes carrying edible French snail" , "French delicacy" , "mollusc, from Paris" , "Hardly fast food" .)