Smile expensive when losing a tooth (7)
I believe the answer is:
grinder
'tooth' is the definition.
(I know that grinder is a type of tooth)
'smile expensive when losing a' is the wordplay.
'smile' becomes 'grin' (synonyms).
'expensive' becomes 'dear' (similar in meaning).
'when losing' is a deletion indicator.
'dear' with 'a' taken out is 'der'.
'grin'+'der'='GRINDER'
(Other definitions for grinder that I've seen before include "Mouth" , "Molar tooth" , "Peppermill" , "Operator of 12 across" , "Mill; molar" .)