Slender, toothless gizmo (9)
I believe the answer is:
thingummy
'gizmo' is the definition.
(I know that gizmo can be written as thingummy)
'slender toothless' is the wordplay.
'slender' becomes 'thin' (synonyms).
'toothless' becomes 'gummy' (I've seen this before).
'thin'+'gummy'='THINGUMMY'
(Other definitions for thingummy that I've seen before include "Whatsit" , "Oojamaflip" , "unknown object" , "what's-its-name" , "Unidentified object" .)