Part of a catch, a special door fastener (4)
I believe the answer is:
hasp
'fastener' is the definition.
(I know that hasp is a type of catch)
'part of a catch a special door' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'HASP' is hidden in the letters of 'catch a special door'.
(Other definitions for hasp that I've seen before include "Hinged fastening" , "part of a lock" , "This part of fastening device" , "catch" , "Fastening plate, clasp" .)