Capital ensemble established in island has not been successful (4,3)
I believe the answer is:
also ran
'has not been successful' is the definition.
(as in an also-ran in a race)
'capital ensemble established in island' is the wordplay.
'capital ensemble' becomes 'LSO' (London Symphony Orchestra).
'established in' indicates putting letters inside.
'island' becomes 'Aran' (as in the Aran Islands).
'lso' put into 'aran' is 'ALSO-RAN'.
(Other definitions for also ran that I've seen before include "last runner?" , "failure?" , "One failing to gain a place" , "Unplaced horse" , "Poor competitor" .)