Sturdy rail knocked over in wild duel (7)
I believe the answer is:
durable
'sturdy' is the definition.
(thesaurus)
'rail knocked over in wild duel' is the wordplay.
'rail' becomes 'bar' (metal rail or bar).
'knocked over' says the letters should be written in reverse.
'in' means one lot of letters goes inside another.
'wild' indicates an anagram.
'bar' back-to-front is 'rab'.
'duel' is an anagram of 'dule'.
'rab' put within 'dule' is 'DURABLE'.
(Other definitions for durable that I've seen before include "Capable of lasting" , "Long-lasting and hard-wearing" , "Tough and long-wearing" , "Likely to last" , "Rugged; long-lasting" .)