In part, I describe watery rises and falls (5)
I believe the answer is:
tides
'rises and falls' is the definition.
(I've seen this in another clue)
'in part i describe watery' is the wordplay.
'in' says the answer is hidden in the clue.
'TIDES' can be found hidden inside 'part i describe watery'.
(Other definitions for tides that I've seen before include "Sited (anag.)" , "Critical moments" , "Those fluctuating" , "Lunar-driven sea movements (5)" , "Diets (anag.)" .)