Essential or practical details (4,3,5)
I believe the answer is:
nuts and bolts
This clue is a double definition.
'essential' is the first definition.
(thesaurus)
'practical details' is the second definition.
(I've seen this before)
(Other definitions for nuts and bolts that I've seen before include "DDessentials of the ironmonger's trade?" , "they often get screwed up" , "Basics - fasteners?" , "available from hardware store" , "Fundamentals" .)