Strong set with nine changes (7)
I believe the answer is:
intense
'strong' is the definition.
(similar in meaning)
'set with nine changes' is the wordplay.
'with' means one lot of letters go next to another.
'changes' indicates anagramming the letters.
'set'+'nine'='setnine'
'setnine' with letters rearranged gives 'INTENSE'.
(Other definitions for intense that I've seen before include "Feverish" , "burning" , "Feeling deeply" , "Strongly-felt" , "Very concentrated or passionate" .)