Frozen water decision partly reversed (3)
I believe the answer is:
ice
'frozen water' is the definition.
'decision partly reversed' is the wordplay.
'partly' says the answer is hidden in the clue.
'reversed' is a reversal indicator.
'eci' is hidden in the letters of 'decision'.
'eci' backwards is 'ICE'.
(Other definitions for ice that I've seen before include "Skating surface" , "dessert" , "Being feeble, can't cut any" , "Constituent of 2dn" , "Diamonds (informal)" .)