Duck swimmer sees partially (7)
I believe the answer is:
immerse
'duck' is the definition.
(I know that duck is a more specific form of the action dip)
'swimmer sees partially' is the wordplay.
'partially' says the answer is hidden in the clue.
'IMMERSE' is hidden in the letters of 'swimmer sees'.
(Other definitions for immerse that I've seen before include "Dunk" , "dive" , "Steep - plunge" , "Cover with water, dip into" , "duck" .)