Parts of the year (days?) involve taking holidays, primarily (6)
I believe the answer is:
months
'parts of the year' is the definition.
'days? involve taking holidays primarily' is the wordplay.
'days?' becomes 'mons' (Mon is an abbreviation for Monday).
'involve' is an insertion indicator.
'primarily' indicates taking the first letters.
The initial letters of 'taking holidays' is 'th'.
'mons' going around 'th' is 'MONTHS'.
(Other definitions for months that I've seen before include "Calendar divisions" , "Divisions of time" , "There are twelve of these" , "Periods of time" , "17 and December" .)