Island insects swarm cross goats (6)
I believe the answer is:
ibexes
'goats' is the definition.
(ibex is a kind of goat)
'island insects swarm cross' is the wordplay.
'island' becomes 'i'.
'insects' becomes 'bees' (bee is a kind of insect).
'swarm' is an insertion indicator (swarm can mean to cover up or smother).
'cross' becomes 'x'.
'bees' placed around 'x' is 'bexes'.
'i'+'bexes'='IBEXES'
(Other definitions for ibexes that I've seen before include "Mountain-dwellers" , "Large wild goats" , "Mountain goats with long curved horns" , "Large mountain goats" , "Wild mountain goats with beards" .)