Expense relating to teeth after incisors half removed (10)
I believe the answer is:
incidental
'expense' is the definition.
(I know that incidental is a type of expense)
'relating to teeth after incisors half removed' is the wordplay.
'relating to teeth' becomes 'dental' (I've seen this before).
'after' says to put letters next to each other.
'half removed' means to take half the letters.
'incisors' with half the letters taken is 'inci'.
'dental' put after 'inci' is 'INCIDENTAL'.
(Other definitions for incidental that I've seen before include "fortuitous" , "By the way" , "form of music" , "Not an essential" .)