Fundamentals for joiners? (4,3,5)
I believe the answer is:
nuts and bolts
I believe this clue is a double definition.
'fundamentals' is the first definition.
(nuts and bolts can means the basics of something)
'joiners?' is the second definition.
(nuts and bolts join things together)
'for' acts as a link.
(Other definitions for nuts and bolts that I've seen before include "Basics - fasteners?" , "Basic essentials (colloq.)" , "Carries out hit-and-run attack" , "Means of securing" , "they often get screwed up" .)