Relative entering that is foolish (5)
I believe the answer is:
inane
'foolish' is the definition.
(similar in meaning)
'relative entering that is' is the wordplay.
'relative' becomes 'nan' (informal term for a grandmother).
'entering' is an insertion indicator.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'nan' placed within 'ie' is 'INANE'.
(Other definitions for inane that I've seen before include "Foolish, silly" , "Lacking sense, silly" , "Silly, witless" , "Thick" , "unimaginative" .)