Declared whopper croc’s cousin a menace to motorists (9)
I believe the answer is:
tailgater
'cousin a menace to motorists' is the definition.
I can't judge whether this defines the answer.
'declared whopper croc's' is the wordplay.
'declared' indicates a 'sounds like' (homophone) clue.
'whopper' becomes 'tale' (I've seen this in another clue).
'croc's' becomes 'gater' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'tale' sounds like 'tail'.
'tail'+'gater'='TAILGATER'
Can you help me to learn more?
(Another definition for tailgater that I've seen is " One closely follows".)