One held up by traffic periodically stopping to modify part of car (3,6)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

I (one) and t R a F f I c (alternate or ‘periodic’ letters) in or ‘stopping’ ALTER (modify) – we weren’t sure about the inclusion of ‘held up’ in the clue – it seemed to be an unnecessary reversal indicator? However, as it is a down clue, it is fair to say that the I is ‘held up’ by RFI – and it makes an excellent surface