Commonly, chap had a mass of hair slicked back with this (6)
I believe the answer is:
pomade
'a mass of hair slicked back with this' is the definition.
The definition suggests a singular noun which matches the answer.
'commonly chap' is the wordplay.
'commonly' becomes 'pom' (I am not sure about this - if you are sure you should believe this answer much more).
'chap' becomes 'Ade'.
'pom'+'ade'='POMADE'
'had' is the link.
(Other definitions for pomade that I've seen before include "preparation for locks" , "Perfumed hair ointment" , "capital application" , "Perfumed hair oil" , "Oily dressing for hair" .)