Greenness of mineral water upset over ends of table (7)
I believe the answer is:
naivete
'greenness' is the definition.
(I've seen this before)
'mineral water upset over ends of table' is the wordplay.
'mineral water' becomes 'evian' (mineral water brand).
'upset' says the letters should be written backwards.
'over' says to put letters next to each other.
'ends of' suggests removing the centre.
'table' with its centre taken out is 'te'.
'evian' in reverse letter order is 'naive'.
'naive'+'te'='NAIVETE'
'of' acts as a link.
(Other definitions for naivete that I've seen before include "Quality of the simple" , "lack of sophistication" , "Innocence or guillibility" , "Lack of worldliness" , "Innocence, artlessness" .)