Some swimmers entering take the plunge (7)
I believe the answer is:
immerse
'the plunge' is the definition.
(thesaurus)
'some swimmers entering take' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'IMMERSE' can be found hidden inside 'swimmers entering take'.
(Other definitions for immerse that I've seen before include "dive" , "duck" , "Plunge, dip, submerge" , "Cover with water, dip into" , "Submerge in water or liquid" .)