Mum's mum wearing that is ridiculous (5)
I believe the answer is:
inane
'ridiculous' is the definition.
(both can mean silly)
'mum's mum wearing that is' is the wordplay.
'mum's mum' becomes 'nan' (I can't justify this - if you can you should give a lot more credence to this answer).
'wearing' indicates putting letters inside.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'nan' going within 'ie' is 'INANE'.
Can you help me to learn more?
(Other definitions for inane that I've seen before include "Idiotic" , "Daft" , "Delirious" , "Foolish, silly" , "Frivolous" .)