Head, smooth, dashes off the basics (4,3,5)
I believe the answer is:
nuts and bolts
'basics' is the definition.
(I've seen this before)
'head smooth dashes' is the wordplay.
'head' becomes 'nut' (informal term for somebody's head).
'smooth' becomes 'sand' (sanding is a kind of smoothing).
'dashes' becomes 'bolts' ('bolt' can be a synonym of 'dash').
'nut'+'sand'+'bolts'='NUTS AND BOLTS'
'off the' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for nuts and bolts that I've seen before include "Basic practical details" , "DDessentials of the ironmonger's trade?" , "Fundamentals" , "main details" , "they often get screwed up" .)