Aridity of a waterless loch (7)
I believe the answer is:
dryness
'aridity' is the definition.
(I've seen this in another clue)
'a waterless loch' is the wordplay.
'a waterless' becomes 'dry' (I've seen this before).
'loch' becomes 'ness' (Loch Ness in Scotland).
'dry'+'ness'='DRYNESS'
'of' acts as a link.
(Other definitions for dryness that I've seen before include "Parched quality" , "Aridity" , "producing thirst" , "Freedom from moisture" , "Matter-of-fact quality (of humour)" .)