Company on small Greek island (3)
I believe the answer is:
cos
'greek island' is the definition.
(I've seen this before)
'company on small' is the wordplay.
'company' becomes 'co' (abbreviation).
'on' is a charade indicator (letters next to each other).
'small' becomes 's' (abbreviation - e.g. clothes size).
'co'+'s'='COS'
(Other definitions for cos that I've seen before include "Variety of lettuce" , "Lettuce with long crisp leaves" , "Senior officers" , "in the Dodecanese?" , "Kind of lettuce in microcosm" .)