Union chaps follow section of pitch (7)
I believe the answer is:
bitumen
'pitch' is the definition.
(thesaurus)
'union chaps follow section' is the wordplay.
'union' becomes 'U' (common abbreviation as in Trades Union Congress).
'chaps' becomes 'men' (chap can mean a man).
'follow' means one lot of letters go next to another.
'section' becomes 'bit' (both can mean part).
'u'+'men'='umen'
'umen' put after 'bit' is 'BITUMEN'.
'of' is the link.
(Other definitions for bitumen that I've seen before include "Road's topping" , "might be used to 16 [RESURFACE]" , "Material for road surfacing, roofing, etc" , "Tarry substance used to surface roads and roofs" , "coverage on the streets?" .)