When one knocks out November, calendar gets less complicated (7)
I believe the answer is:
plainer
'less complicated' is the definition.
'when one knocks out november calendar' is the wordplay.
I cannot quite see how this works, but
'when' could be 'e' (I've seen this in other clues) and 'e' is present in the answer.
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
'november' could be 'n' (phonetic alphabet: alpha, bravo, charlie etc.) and 'n' is found in the answer.
This may be the basis of clue (or it may be nonsense).
'gets' acts as a link.
(Other definitions for plainer that I've seen before include "Simpler kind of praline" , "More obvious, less decorated" .)