Prospective water carrier, quarter-pint, she said (4)
I believe the answer is:
jill
'prospective water carrier' is the definition.
(ref to Jack and Jill nursery rhyme)
I can't explain the rest of the clue.
(Other definitions for jill that I've seen before include "Jack's nursery rhyme girl companion" , "Jack's pet" , "Female ferret" .)