Particular calendar one received, kept by Harry in retirement (9)
I believe the answer is:
gregorian
'particular calendar' is the definition.
'Gregorian' can be an answer for 'calendar' (Gregorian calendar is an example). I am not sure about the 'particular' bit.
'one received kept by harry in retirement' is the wordplay.
'one' becomes 'i' (Roman numeral).
'received' becomes 'roger' (I've seen this before).
'kept by' means one lot of letters goes inside another.
'harry' becomes 'nag' (both can mean to consistently bother).
'in retirement' says the letters should be written in reverse (retire can mean to go back or backwards).
'i'+'roger'='iroger'
'iroger' placed within 'nag' is 'nairogerg'.
'nairogerg' back-to-front is 'GREGORIAN'.
Can you help me to learn more?
(Other definitions for gregorian that I've seen before include "maybe chants?" , "Sort of chant" , "Like sixteen popes" , "Type of calendar, of chant" , "relating to calendar" .)