Cost's small including new Queen's daughter? (8)
I believe the answer is:
princess
'queen's daughter?' is the definition.
(a daughter of a monarch is a princess)
'cost's small including new' is the wordplay.
'cost's' becomes 'price's' (the cost or price of something).
'small' becomes 's' (abbreviation - e.g. clothes size).
'including' means one lot of letters goes inside another.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'prices'+'s'='pricess'
'pricess' going around 'n' is 'PRINCESS'.
(Other definitions for princess that I've seen before include "Part of royalty" , "Royal girl" , "VIP" , "Daughter of a sovereign" , "Royal title" .)