The writer's using short pens for statement (9)
I believe the answer is:
testimony
'statement' is the definition.
(testimony is a kind of statement)
'the writer's using short pens' is the wordplay.
I cannot quite understand how this works, but
'writer's' could be 'im' ('writer's' could mean 'the crossword setter is') and 'im' is found within the answer.
This may be the basis of clue (or it may be nonsense).
'for' acts as a link.
(Other definitions for testimony that I've seen before include "Formal statement" , "Evidence given in court" , "Evidence, declaration" , "witness statement" , "Evidence offered by a witness" .)