Extremely unusual to neglect going round last month (6)
I believe the answer is:
ultimo
'last month' is the definition.
(I've seen this before)
'extremely unusual to neglect going round' is the wordplay.
'extremely' says to hollow out the word (remove centre letters).
'to neglect' becomes 'omit' ('omit' can be a synonym of 'neglect').
'going round' says the letters should be written in reverse.
'unusual' with its middle removed is 'ul'.
'omit' backwards is 'timo'.
'ul'+'timo'='ULTIMO'
(Other definitions for ultimo that I've seen before include "Indicating last month, usually abbreviated" , "Previous month" , "In commerce, of last month" , "In full, last month in business correspondence" , "(Dated) of last month" .)