Vessel always must keep covered in festival (7,3)
I believe the answer is:
pancake day
'festival' is the definition.
'vessel always must keep covered' is the wordplay.
'vessel' becomes 'pan' (I have seen 'Shallow vessel ' mean 'pan' so perhaps 'vessel' could also mean 'pan').
'always' becomes 'ay'.
'must keep' is an insertion indicator.
'covered' becomes 'caked' (caking is a kind of covering**).
'pan'+'ay'='panay'
'panay' going around 'caked' is 'PANCAKE DAY'.
'in' is the link.
(Other definitions for pancake day that I've seen before include "Shrove Tuesday" , "Shrove Tues. (colloq.)" .)