Watering hole, old, unaltered (5)
I believe the answer is:
oasis
'watering hole' is the definition.
(I've seen this before)
'old unaltered' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'unaltered' becomes 'asis' (I've seen this in other clues).
'o'+'asis'='OASIS'
(Other definitions for oasis that I've seen before include "Fertile patch in a desert" , "Manchester band featuring the Gallagher brothers" , "Fertile area in desert" , "Desert pool" , "Desert refuge" .)