Against American behind abridgement of poetry (6)
I believe the answer is:
versus
'against' is the definition.
(as in one sports team versus another)
'american behind abridgement of poetry' is the wordplay.
'american' becomes 'us' (abbreviation for United States).
'behind' means one lot of letters go next to another.
'abridgement of' means to remove the last letter (to abridge is to shorten).
'poetry' becomes 'verse' (synonyms).
'verse' with its last letter taken away is 'vers'.
'us' put after 'vers' is 'VERSUS'.
(Other definitions for versus that I've seen before include "Against (opponent)" , "v. (sport)" , "Set against" , "taking part in team sports" , "In opposition to" .)