Lengthen for the page on tractor assembly (8)
I believe the answer is:
protract
'lengthen' is the definition.
(protracting is a kind of lengthening)
'the page on tractor assembly' is the wordplay.
'the page' becomes 'p'.
'on' means one lot of letters go next to another.
'assembly' is an anagram indicator.
'tractor' anagrammed gives 'rotract'.
'p'+'rotract'='PROTRACT'
'for' is the link.
(Other definitions for protract that I've seen before include "Drag out at length" , "Draw out, lengthen the time" , "Extend length" , "Increase" , "Draw out, prolong" .)