As a swimmer, could start to do badly! (3)
I believe the answer is:
cod
'as a swimmer' is the definition.
I can't judge whether this definition defines the answer.
'could start to do badly' is the wordplay.
'start to' suggests taking the first letters.
'badly' is an anagram indicator (letters in the wrong order).
The first letter of 'could' is 'c'.
'do' with letters rearranged gives 'od'.
'c'+'od'='COD'
Can you help me to learn more?
(Other definitions for cod that I've seen before include "Kid" , "A fish; a hoax (slang)" , "Large sea fish- it's a sham" , "Cash on delivery (abbr.)" , "Payment arrangement" .)