Strong pet not tamed (6)
I believe the answer is:
potent
'strong' is the definition.
('potent' can be a synonym of 'strong')
'pet not tamed' is the wordplay.
'tamed' is an anagram indicator.
'pet'+'not'='petnot'
'petnot' anagrammed gives 'POTENT'.
(Other definitions for potent that I've seen before include "Powerful or effective" , "Having great power" , "Intoxicating" , "Strong in operation" , "Mighty" .)