Straightforward writer in thousand, possibly (4-3-4)
I believe the answer is:
open-and-shut
'straightforward' is the definition.
(I've seen this before)
'writer in thousand possibly' is the wordplay.
'writer' becomes 'pen' (something which writes).
'in' indicates putting letters inside.
'possibly' indicates anagramming the letters.
'thousand' is an anagram of 'oandshut'.
'pen' put inside 'oandshut' is 'OPEN-AND-SHUT'.
(Other definitions for open-and-shut that I've seen before include "So obvious as to be easily decided" , "No nude paths (anag) - easily decided" , "Straightforward" .)