Express on reflection singer's nothing to regret (8)

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

Express , on reflection, singer’s nothing to regret RAT’S (=singer’s, i.e. informer, traitor) + O (=nothing, pictorially) + RUE (=to regret); Eurostar is a high-speed star, hence “express”; “on reflection” indicates reversal