Satisfies thirst — not principally in their water? (5)
I believe the answer is:
lakes
'their water?' is the definition.
(bodies of water)
'satisfies thirst not principally' is the wordplay.
'satisfies thirst' becomes 'slakes' (to slake one's thirst is to satisfy it).
'not principally' means to remove the first letter (the principal letter is removed).
'slakes' with its initial letter taken off is 'LAKES'.
'in' acts as a link.
(Other definitions for lakes that I've seen before include "Bodies of (usually fresh) water" , "Enclosed areas of water" , "Large bodies of water surrounded by land" .)