Fly Mike over small South American capital (8)
I believe the answer is:
mosquito
'fly' is the definition.
(I've seen this before)
'mike over small south american capital' is the wordplay.
'mike' becomes 'M' (phonetic alphabet: alpha, bravo, charlie etc.).
'over' becomes 'o' (cricket abbreviation).
'small' becomes 's' (abbreviation - e.g. clothes size).
'south american capital' becomes 'quito' (capital of Ecuador).
'm'+'o'+'s'+'quito'='MOSQUITO'
(Other definitions for mosquito that I've seen before include "Kind of blood-sucking tropical gnat" , "British WWII multi-role combat aircraft" , "Insect that transmits malaria" , "Malaria-carrying insect" , "Biting bug" .)