A reason against having the name in pencil (6)
I believe the answer is:
crayon
'pencil' is the definition.
(I've seen this before)
'a reason against' is the wordplay.
'a reason' becomes 'cray' (I am not sure about this - if you are sure you should believe this answer much more).
'against' becomes 'on' (I've seen this in other clues).
'cray'+'on'='CRAYON'
'having the name in' 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 crayon that I've seen before include "Writer" , "Cyrano (anag)" , "Wax colouring stick" , "Colouring pencil" , "pencil (13) [FRENCH]" .)