Defiant Iran tests resolution (9)
I believe the answer is:
resistant
'defiant' is the definition.
('resistant' can be similar in meaning to 'defiant')
'iran tests resolution' is the wordplay.
'resolution' indicates anagramming the letters.
'iran'+'tests'='irantests'
'irantests' anagrammed gives 'RESISTANT'.
(Other definitions for resistant that I've seen before include "Being hostile" , "repellent" , "holding out" , "unaffected" , "Hindering the conductor" .)