Leaders in South American republic eat fish (7)
I believe the answer is:
sardine
'fish' is the definition.
(sardine is a type of fish)
'leaders in south american republic eat' is the wordplay.
'leaders in' suggests taking the first letters.
'eat' becomes 'dine' (to dine is to eat a meal).
The initial letters of 'south american republic' is 'sar'.
'sar'+'dine'='SARDINE'
(Other definitions for sardine that I've seen before include "Small fish, usually canned" , "Rinsed a (anag) - fish" , "A diner's small fish" , "Fish in tin?" , "Stonefish OR? Stone" .)