As a vehicle for verse (8)
I believe the answer is:
quatrain
'verse' is the definition.
(thesaurus)
'as a vehicle' is the wordplay.
'as' becomes 'qua' (Latin word meaning 'in the role of', 'as').
'a vehicle' becomes 'train'.
'qua'+'train'='QUATRAIN'
'for' acts as a link.
(Other definitions for quatrain that I've seen before include "Type of verse with four lines" , "poetic lines" , "Poem of four lines found in a quart" , "Poetry" , "Poet may write it" .)