Duck is some swimmer, seriously! (7)
I believe the answer is:
immerse
'duck' is the definition.
(I know that duck is a more specific form of the action dip)
'some swimmer seriously' is the wordplay.
'some' says the answer is hidden in the clue (some parts of the word are used).
'IMMERSE' can be found hidden inside 'swimmer seriously'.
'is' acts as a link.
(Other definitions for immerse that I've seen before include "dive" , "Submerge in water or liquid" , "Drown" , "Plunge, dip, submerge" , "Steep - plunge" .)