Oblique American supporting firm (7)
I believe the answer is:
solidus
'oblique' is the definition.
(thesaurus)
'american supporting firm' is the wordplay.
'american' becomes 'us'.
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'firm' becomes 'solid' ('solid' can be a synonym of 'firm'**).
'us' put after 'solid' is 'SOLIDUS'.
(Other definitions for solidus that I've seen before include "old coin" , "Slash symbol" , "Oblique stroke, slash" .)