Make wavy cover for something straight each day (3,4)
I believe the answer is:
per diem
'each day' is the definition.
(I've seen this before)
'make wavy cover for something straight' is the wordplay.
'make wavy' becomes 'perm'.
'cover for' indicates putting letters inside.
'something straight' becomes 'die' (straight as a die).
'perm' going around 'die' is 'PER DIEM'.
(Other definitions for per diem that I've seen before include "living expenses" , "For each day (Latin)" , "By the day, to the old Romans" .)