Grid where every other clue involves classical Greek (7)
I believe the answer is:
lattice
'grid' is the definition.
(I've seen this before)
'every other clue involves classical greek' is the wordplay.
'every other' means one should take alternating letters.
'involves' means one lot of letters goes inside another.
'classical greek' becomes 'attic' (synonyms).
The alternating letters of 'clue' are 'le'.
'le' going around 'attic' is 'LATTICE'.
'where' is the link.
(Other definitions for lattice that I've seen before include "Ornamental framework" , "Regular atomic structure" , "grill" , "Criss-cross pattern" , "Open framework in pattern" .)