Aegean island some seek ostentatiously (3)
I believe the answer is:
kos
'aegean island' is the definition.
(I've seen this before)
'some seek ostentatiously' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'KOS' can be found hidden inside 'seek ostentatiously'.
(Other definitions for kos that I've seen before include "Island of the Dodecanese" , "Greek island" , "Aegean island" , "Aegean isle" .)