Vacuous grandmother in Nimes regularly (5)
I believe the answer is:
inane
'vacuous' is the definition.
(synonyms)
'grandmother in nimes regularly' is the wordplay.
'grandmother' becomes 'nan' (nan can informally mean a grandmother).
'in' means one lot of letters goes inside another.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternate letters of 'nimes' are 'ie'.
'nan' put into 'ie' is 'INANE'.
(Other definitions for inane that I've seen before include "Devoid of intelligence" , "Absurd" , "Witless and pointless" , "Stupid and silly" , "Vacuous, silly" .)