Spare toothless whatsit! (9)
I believe the answer is:
thingummy
'whatsit' is the definition.
(both can mean an unspecified object)
'spare toothless' is the wordplay.
'spare' becomes 'thin' (similar in meaning).
'toothless' becomes 'gummy' (I've seen this before).
'thin'+'gummy'='THINGUMMY'
(Other definitions for thingummy that I've seen before include "Unidentified object" , "whose name one's forgotten" , "what's-its-name" , "Oojamaflip" , "Whatsit" .)