Geometrician gives leaders of European Union countries some cover (6)
I believe the answer is:
euclid
'geometrician' is the definition.
(Euclid is an example)
'leaders of european union countries some cover' is the wordplay.
'leaders of' suggests taking the first letters.
'some cover' becomes 'lid' (lid is a kind of cover).
The initial letters of 'european union countries' is 'euc'.
'euc'+'lid'='EUCLID'
'gives' acts as a link.
(Other definitions for euclid that I've seen before include "Ancient Greek mathematician" , "Greek father of geometry" , "Greek mathematician, 3rd century BC" , "Old Greek master of geometry" , "mathematician appears" .)