Efficient island uprising (4)
I believe the answer is:
able
'efficient' is the definition.
(I've seen this before)
'island uprising' is the wordplay.
'island' becomes 'elba' (I've seen this before).
'uprising' is a reversal indicator.
'elba' reversed gives 'ABLE'.
(Other definitions for able that I've seen before include "Fit as a seaman" , "Capable seaman" , "Qualified - expert" , "... was I ere I saw Elba (palindrome)" , "Fully competent" .)