Rice dish - mum’s invested in outfit almost completely (5)
I believe the answer is:
sushi
'rice dish' is the definition.
(sushi is a type of rice dish)
'mum's invested in outfit almost completely' is the wordplay.
'mum' becomes 'sh' (both can mean to be quiet).
'invested in' indicates putting letters inside.
'outfit' becomes 'suit' (suit is a type of outfit).
'almost completely' means to remove the last letter.
'suit' with its last letter removed is 'sui'.
'sh' inserted inside 'sui' is 'SUSHI'.
(Other definitions for sushi that I've seen before include "Exotic food" , "put away in Japan" , "Asian dish" , "Japanese cuisine" , "Japanese vinegared rice dish" .)