Water, perhaps - a small amount (7)
I believe the answer is:
element
This clue is a double definition.
'water perhaps' is the first definition.
(water is a kind of element)
'a small amount' is the second definition.
(I've seen this before)
(Other definitions for element that I've seen before include "Ideal situation for someone" , "that's used in heating" , "Singular ingredient" , "eg Fire, air" , "silver perhaps?" .)