Within the French garret, a grille (7)
I believe the answer is:
lattice
'a grille' is the definition.
(I know that grille can be written as lattice)
'within the french garret' is the wordplay.
'within' is an insertion indicator.
'the french' becomes 'le' ('the' in French).
'garret' becomes 'attic' (synonyms).
'le' placed around 'attic' is 'LATTICE'.
(Other definitions for lattice that I've seen before include "Network of crossed laths" , "grill" , "Arrangement with bars" , "Regular open structure" , "type of tracery" .)