Detach coreless operating system (4)
I believe the answer is:
unix
'operating system' is the definition.
(Unix is an operating system)
'detach coreless' is the wordplay.
'detach' becomes 'unfix' (I've seen this before).
'coreless' suggests removing the centre.
'unfix' with its centre removed is 'UNIX'.
(Other definitions for unix that I've seen before include "See this operating system" , "software" , "Computer open operating system" .)