Ringing grandma, that is pointless (5)
I believe the answer is:
inane
'pointless' is the definition.
(I've seen this in another clue)
'ringing grandma that is' is the wordplay.
'ringing' means one lot of letters goes inside another (some letters go round or 'ring' others).
'grandma' becomes 'nan' (nan can informally mean a grandmother).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'nan' put within 'ie' is 'INANE'.
(Other definitions for inane that I've seen before include "Thick" , "Idiotic" , "Empty-headed" , "Silly, witless" , "Foolish, silly" .)