Thingummy with a hollow shaft, broken (7)
I believe the answer is:
whatsit
'thingummy' is the definition.
(I know that thingummy can be written as whatsit)
'with a hollow shaft broken' is the wordplay.
'hollow' means to remove the middle letters (the word is hollowed out).
'broken' indicates anagramming the letters.
'shaft' with its middle taken out is 'st'.
'with'+'a'+'st'='withast'
'withast' anagrammed gives 'WHATSIT'.
(Other definitions for whatsit that I've seen before include "person not to be named" , "Thingumajig" , "something indefinable" , "Thingummy" , "Something one can't recall" .)