Heavenly bodies encountered in island (6)
I believe the answer is:
comets
'heavenly bodies' is the definition.
(comet is a kind of heavenly body)
'encountered in island' is the wordplay.
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'in' indicates putting letters inside.
'island' becomes 'cos' (Greek island).
'met' placed into 'cos' is 'COMETS'.
(Other definitions for comets that I've seen before include "Small extraterrestrial bodies" , "Heavenly bodies with tails" , "Space travellers" .)