As a swimmer, could start to do badly! (3)
I believe the answer is:
cod
'as a swimmer' is the definition.
I can't tell whether this defines the answer.
'could start to do badly' is the wordplay.
'start to' says to take the initial letters.
'badly' is an anagram indicator (letters in the wrong order).
The first letter of 'could' is 'c'.
'do' anagrammed 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" .)