Old unaltered watering hole (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 "Spring of fresh water in a desert" , "Peaceful area away from trouble" , "A place of safety or sanctuary" , "flower arranging stuff" , "place of refuge" .)