Outstanding concert in Argentina's centre (9)
I believe the answer is:
prominent
'outstanding' is the definition.
(synonyms)
'concert in argentina's centre' is the wordplay.
'concert' becomes 'prom' (abbreviation for promenade concert).
'centre' says to take the centre.
The middle letters of 'argentina' are 'ent'.
'prom'+'in'+'ent'='PROMINENT'
(Other definitions for prominent that I've seen before include "Conspicuous in position or importance" , "Standing out" , "Projecting; famous" , "Well-known" , "Important, famous" .)