Clue: Commonly, chap had a mass of hair slicked back with this (6)
Answer: POMADE
Explanation: Definition: with this
*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-