Wash hair in seawater then trim (5)
I believe the answer is:
rinse
'wash' is the definition.
(rinsing is a kind of washing)
'hair in seawater then trim' is the wordplay.
'trim' indicates a hidden word (I've seen 'trimmed' mean this (part of the clue must be trimmed away)).
'RINSE' is hidden in the letters of 'hair in seawater then'.
(Other definitions for rinse that I've seen before include "Run water through" , "Wash off surface dirt" , "Sluice" , "blue one on top?" , "Wash lightly without soap" .)