Granny is being kept inside for a month (5)
I believe the answer is:
nisan
'for a month' is the definition.
(month of the Hebrew calendar)
'granny is being kept' is the wordplay.
'granny' becomes 'nan'.
'being kept' is an insertion indicator.
'nan' enclosing 'is' is 'NISAN'.
'inside' acts as a link.
(Other definitions for nisan that I've seen before include "month in Israel" , "when Passover begins" .)