Constituent of tar: part primarily used by people (7)
I believe the answer is:
bitumen
'constituent of tar' is the definition.
(tar includes bitumen)
'part primarily used by people' is the wordplay.
'part' becomes 'bit' (both mean a piece of something).
'primarily' indicates taking the first letters.
'by' is a charade indicator (letters next to each other).
'people' becomes 'men' (men can refer to humanity as a whole).
The first letter of 'used' is 'u'.
'bit'+'u'+'men'='BITUMEN'
(Other definitions for bitumen that I've seen before include "Road's topping" , "Road-surfacing material" , "Material for road surfacing, roofing, etc" , "Substance for surfacing roads" , "material that surfaces" .)