Mostly stupid new Sunday exercises (3-3)
I believe the answer is:
sit ups
'exercises' is the definition.
(I've seen this before)
'mostly stupid new sunday' is the wordplay.
'mostly' means to remove the last letter.
'new' is an anagram indicator.
'sunday' becomes 's'.
'stupid' with its final letter taken off is 'stupi'.
'stupi' anagrammed gives 'situp'.
'situp'+'s'='SIT-UPS'
(Other definitions for sit ups that I've seen before include "physical exercise?" , "exercises" .)