Alan: chap with a century calendar (7)
I believe the answer is:
almanac
'calendar' is the definition.
(I've seen this before)
'alan chap with a century' is the wordplay.
'alan chap' becomes 'alma' (I am not sure about this - if you are sure you should believe this answer much more).
'with' is an insertion indicator.
'a' becomes 'an'.
'century' becomes 'C' (common abbreviation).
'alma' placed around 'an' is 'almana'.
'almana'+'c'='ALMANAC'
Can you help me to learn more?
(Other definitions for almanac that I've seen before include "Year book with astronomical events, tides etc" , "periodic publication" , "Annual publication containing information in a particular field" , "Calendar with astronomical data and other information" , "Annual publication with mixed data, forecasts etc." .)