Tar, substance sealing top of carriageway (5)
I believe the answer is:
pitch
'tar' is the definition.
(synonyms)
'substance sealing top of carriageway' is the wordplay.
'substance' becomes 'pith' (both can mean something's essence or gist).
'sealing' indicates putting letters inside (inserted letters are sealed inside).
'top of' indicates taking the first letters.
The first letter of 'carriageway' is 'c'.
'pith' placed around 'c' is 'PITCH'.
(Other definitions for pitch that I've seen before include "Playing field as for football" , "Hurl" , "Level of sound" , "Tarry stuff for football field?" , "Timbre" .)