Lactose mix-up before meal ends in spew (5)
I believe the answer is:
expel
'spew' is the definition.
(I've seen this before)
'lactose mix-up before meal ends' is the wordplay.
'ends' says to take the final letters.
The last letters of 'lactose mix up before meal' are 'EXPEL'.
'in' acts as a link.
(Other definitions for expel that I've seen before include "Kick out" , "Possibly remove from club" , "Put out" , "Deport" , "Get rid" .)