A reservoir's a lake, though empty of water (5,3)
I believe the answer is:
adams ale
'water' is the definition.
(humorous term for water)
'a reservoir's a lake though empty' is the wordplay.
I cannot really see how this works, but
'a' is found in the answer.
'lake' could be 'l' (geographical abbreviation) and 'l' is found within the answer.
'empty' could be 'e' (abbreviation e.g. on guages) and 'e' is found within the answer.
This explanation may well be incorrect...
'of' is the link.
Can you help me to learn more?
(Other definitions for adams ale that I've seen before include "Drink - a sad meal (anag)" , "Water (humorous)" , "Liquid refreshment" , "simple drink" , "water instead" .)