Lengthen pamphlet after person making regular cuts (8)
I believe the answer is:
protract
'lengthen' is the definition.
(I know that protract is a more specific form of the action lengthen)
'pamphlet after person making regular cuts' is the wordplay.
'pamphlet' becomes 'tract' (synonyms).
'after' is a charade indicator (letters next to each other).
'making regular cuts' indicates alternate letters.
The alternate letters of 'person' are 'pro'.
'tract' after 'pro' is 'PROTRACT'.
(Other definitions for protract that I've seen before include "Extend length" , "Drag out at length" , "Increase" , "Draw out, prolong" , "Stretch out" .)