Mechanics might need to secure these basic details (4,3,5)
I believe the answer is:
nuts and bolts
I believe this clue is a double definition.
'mechanics might need to secure these' is the first definition.
(mechanics might use nuts and bolts)
'basic details' is the second definition.
(I've seen this before)
(Other definitions for nuts and bolts that I've seen before include "main details" , "Carries out hit-and-run attack" , "brass tacks?" , "DDessentials of the ironmonger's trade?" , "Means of securing" .)