Wonderful agent, American, turning up (5)
I believe the answer is:
super
'wonderful' is the definition.
(I've seen this before)
'agent american turning up' is the wordplay.
'agent' becomes 'rep' (short for representative).
'american' becomes 'us' (abbreviation for United States).
'turning up' says the letters should be written in reverse.
'rep'+'us'='repus'
'repus' reversed gives 'SUPER'.
(Other definitions for super that I've seen before include "Excellent - small-part actor" , "Excellent; police officer" , "(Of product) of extra fine quality" , "Excellent; policeman" , "Extremely good" .)