Main fix (6)
I believe the answer is:
staple
I believe this is a double definition.
'main' is the first definition.
(thesaurus)
'fix' is the second definition.
(stapling is a kind of fixing)
(Other definitions for staple that I've seen before include "security device" , "Main - way to fasten paper" , "Clip" , "Fundamental" , "means of keeping papers together" .)