Contaminate fin, etc, when swimming (6)
I believe the answer is:
infect
'contaminate' is the definition.
(infecting is a kind of contaminating)
'fin etc when swimming' is the wordplay.
'when swimming' indicates an anagram.
'fin'+'etc'='finetc'
'finetc' with letters rearranged gives 'INFECT'.
(Other definitions for infect that I've seen before include "Contaminate with disease" , "Give (someone) a disease" , "Affect (with a virus)" , "Transmit disease to another" , "Contaminate, make ill" .)