Thingamy (6)
I believe the answer is:
doodah
'thingamy' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for doodah that I've seen before include "Gizmo, thingamabob" , "Thingummy" , "Ornament" , "Thing whose name you can't remember" , "Object when someone forgets" .)