Source of deplorable traffic noise we heard periodically (9)
I believe the answer is:
racketeer
'source of deplorable traffic' is the definition.
(someone who traffics or sells dishonestly)
'noise we heard periodically' is the wordplay.
'noise' becomes 'racket' (synonyms).
'periodically' indicates alternate letters (letters must be selected 'every so often').
'we'+'heard'='weheard'
The alternating letters of 'weheard' are 'eer'.
'racket'+'eer'='RACKETEER'
(Other definitions for racketeer that I've seen before include "who knows way round court, you might say?" , "fraudster" , "tennis player?" , "Wide boy" , "dishonest dealer" .)