Defiant as train set's broken (9)
I believe the answer is:
resistant
'defiant' is the definition.
('resistant' can be similar in meaning to 'defiant')
'train set's broken' is the wordplay.
'broken' is an anagram indicator.
'train'+'sets'='trainsets'
'trainsets' anagrammed gives 'RESISTANT'.
'as' is the link.
(Other definitions for resistant that I've seen before include "Impervious (to corrosion, heat, etc)" , "tough" , "Withstanding adverse conditions" , "unlikely to be damaged?" , "Hindering the conductor" .)