Supportive framework required, test on horse backfiring (6)
I believe the answer is:
gantry
'supportive framework' is the definition.
'gantry' can be an answer for 'framework' (gantry is a kind of framework). I also know that 'supportive' relates to this answer.
'test on horse backfiring' is the wordplay.
'test' becomes 'try' (synonyms).
'on' says to put letters next to each other.
'horse' becomes 'nag' (term for an old horse).
'backfiring' shows that the letters should be reversed in order.
'nag' in reverse letter order is 'gan'.
'try' put after 'gan' is 'GANTRY'.
'required' acts as a link.
(Other definitions for gantry that I've seen before include "Signal support" , "Metal framework for crane, signals, etc" , "Bridging framework to support crane, signals etc" , "Bridge-like framework for moblile crane" , "Support for, eg, crane" .)