Nuts and bolts in pointer on motorway (6)
I believe the answer is:
marrow
'nuts and bolts' is the definition.
(thesaurus)
'pointer on motorway' is the wordplay.
'pointer' becomes 'arrow' (synonyms).
'on' means one lot of letters go next to another.
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
'arrow' after 'm' is 'MARROW'.
'in' acts as a link.
(Other definitions for marrow that I've seen before include "Vegetable - tissue in bone" , "Matter inside bones, or gourd fruit" , "one of two" , "Stuff found inside bones" , "The heart" .)