Design area to host mass meal (5,3)
I believe the answer is:
cream tea
'meal' is the definition.
(I've seen this before)
'design area to host mass' is the wordplay.
'design' becomes 'create' (I've seen this before).
'area' becomes 'a' (maths abbreviation).
'to host' indicates putting letters inside (inserted letters are hosted or welcomed).
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'create'+'a'='createa'
'createa' enclosing 'm' is 'CREAM TEA'.
(Other definitions for cream tea that I've seen before include "Meal with scones and jam" , "Devon speciality" , "Light afternoon meal" , "Supposedly civilised afternoon meal" , "posh food" .)