Smile, darling, although missing a tooth (7)
I believe the answer is:
grinder
'tooth' is the definition.
(grinder is a kind of tooth)
'smile darling although missing a' is the wordplay.
'smile' becomes 'grin' (grinning is a kind of smiling).
'darling' becomes 'dear' ('dear' can be a synonym of 'darling').
'although missing' is a deletion indicator.
'dear' with 'a' taken away is 'der'.
'grin'+'der'='GRINDER'
(Other definitions for grinder that I've seen before include "Eg, pepper mill" , "Mouth" , "Molar tooth" , "Salt/pepper mill" , "Operator of 12 across" .)