Mum in studio regularly selected Oriental food (5)
I believe the answer is:
'oriental food' is the definition.
(I've seen this before)
'mum in studio regularly selected' is the wordplay.
'mum' becomes 'sh' (both are used to tell someone to be quiet).
'in' means one lot of letters goes inside another.
'regularly selected' indicates alternate letters.
The alternating letters of 'studio' are 'sui'.
'sh' put within 'sui' is 'SUSHI'.
(Other definitions for sushi that I've seen before include "Japanese cold rice dish" , "Japanese cuisine" , "Oriental food" , "Asian dish" , "cold food" .)