What a pity, I included assumed name (5)
I believe the answer is:
alias
'assumed name' is the definition.
('alias' can be a synonym of 'assumed name')
'what a pity i included' is the wordplay.
'what a pity' becomes 'alas' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'included' means one lot of letters goes inside another.
'alas' enclosing 'i' is 'ALIAS'.
Can you help me to learn more?
(Other definitions for alias that I've seen before include "it may be assumed" , "stage name, perhaps" , "Handle that isn't genuine" , "Otherwise known as" , "aka" .)