Essentials, those used in construction (4,3,5)
I believe the answer is:
nuts and bolts
I believe this clue is a double definition.
'essentials' is the first definition.
'those used in construction' is the second definition.
(Other definitions for nuts and bolts that I've seen before include "Basic essentials (colloq.)" , "A hardware shop should have them" , "Essential basics" , "Basics - fasteners?" , "Means of securing" .)