Filter, at most, half the water (6)
I believe the answer is:
strain
'filter' is the definition.
('strain' can be a synonym of 'filter')
'at most half the water' is the wordplay.
'at' says to put letters next to each other.
'half' indicates taking half.
'the water' becomes 'rain' (rain is a kind of water**).
'most' halved is 'st'.
'st' next to 'rain' is 'STRAIN'.
(Other definitions for strain that I've seen before include "Make intensive effort" , "Pass through a filter" , "Muscle injury, put liquid through sieve" , "'Stress, tension (6)'" , "Screen" .)