Enthusiastically changing gear in city (7)
I believe the answer is:
eagerly
'enthusiastically' is the definition.
(thesaurus)
'changing gear in city' is the wordplay.
'changing' is an anagram indicator.
'in' indicates putting letters inside.
'city' becomes 'ely' (I've seen this before).
'gear' with letters rearranged gives 'ager'.
'ager' going inside 'ely' is 'EAGERLY'.
(Other definitions for eagerly that I've seen before include "anxiously" , "Fervently" , "In a keen manner" , "With keen anticipation" , "With hunger" .)