Commonly, chap had a mass of hair slicked back with this (6)

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

Edit: Widdersbel has a better explanation for the first part of the wordplay: HE’D=he had=”chap had”, dropping the ‘h’ as indicated by “Commonly” ED (short for e.g. Edward, “chap”) + A MOP=”a mass of hair”, all reversed/”back” “Commonly” indicates the shortening of Ed-