To uncle, a very large Aegean island (5)
I believe the answer is:
samos
'aegean island' is the definition.
(I've seen this before)
'to uncle a very large' is the wordplay.
'to uncle' becomes 'sam' (Uncle Sam is an example).
'a very large' becomes 'os' (abbreviation for oversize).
'sam'+'os'='SAMOS'
(Other definitions for samos that I've seen before include "Aegean Sea island" , "somewhere in the Aegean" , "birthplace of Pythagoras" , "Greek island?" .)