Swimmer over the top, I hesitate to say (5)
I believe the answer is:
otter
'swimmer' is the definition.
(animal which swims)
'over the top i hesitate to say' is the wordplay.
'over the top' becomes 'OTT' (abbreviation).
'i hesitate to say' becomes 'er' (expression used when hesitating).
'ott'+'er'='OTTER'
(Other definitions for otter that I've seen before include "Fish-eater of the weasel family" , "Something furry" , "Aquatic river mammal" , "Aquatic beast" , "carnivore is concerned" .)