Island insects swarm cross goats (6)
I believe the answer is:
'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'.
(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" .)