The writer is vehicle for poetic form (6)
I believe the answer is:
iambus
'for poetic form' is the definition.
(I've seen this before)
'the writer is vehicle' is the wordplay.
'the writer is' becomes 'iam' (I can't justify this - if you can you should give a lot more credence to this answer).
'vehicle' becomes 'bus' (public transport vehicle).
'iam'+'bus'='IAMBUS'
Can you help me to learn more?
(Other definitions for iambus that I've seen before include "Type of metrical unit in a poem" , "Metrical unit of two syllables" , "poetic form" , "Metrical foot" .)