Felt ruthless somewhat picking up swimmer (6)
I believe the answer is:
turtle
'swimmer' is the definition.
(I've seen this before)
'felt ruthless somewhat picking up' is the wordplay.
'somewhat' indicates a hidden word.
'picking up' shows that the letters should be reversed in order.
'eltrut' is hidden within 'felt ruthless'.
'eltrut' backwards is 'TURTLE'.
(Other definitions for turtle that I've seen before include "Neckline and reptile" , "Shelled reptile" , "Birds typifying love" , "See 7" , "Marine reptile of the order Chelonia" .)