Green quality of water from South Tyrone's borders (7)
I believe the answer is:
naivete
'green quality' is the definition.
(I know this)
'water from south tyrone's borders' is the wordplay.
'water' becomes 'evian' (mineral water brand).
'from south' is a reversal indicator.
'borders' suggests removing the centre (only the edge or border letters remain).
'tyrone' with its middle taken out is 'te'.
'evian' reversed gives 'naive'.
'naive'+'te'='NAIVETE'
'of' is the link.
(Other definitions for naivete that I've seen before include "Being green" , "lack of experience" , "natural simplicity" , "State of inexperience (French spelling)" , "lack of sophistication" .)