Stubborn playing with train sets (9)
I believe the answer is:
resistant
'stubborn' is the definition.
(thesaurus)
'playing with train sets' is the wordplay.
'playing' is an anagram indicator.
'with' is a charade indicator (letters next to each other).
'train'+'sets' is 'trainsets'.
'trainsets' is an anagram of 'RESISTANT'.
(Other definitions for resistant that I've seen before include "tough" , "repellent" , "Impervious (to corrosion, heat, etc)" , "Able to withstand adverse conditions" , "Not affected by something, a bug by an antibiotic say" .)