Benign power supply taken under a road (8)
I believe the answer is:
amicable
'benign' is the definition.
(thesaurus)
'power supply taken under a road' is the wordplay.
'power supply' becomes 'cable'.
'taken under' says to put letters next to each other.
'road' becomes 'mi'.
'a'+'mi'='ami'
'cable' after 'ami' is 'AMICABLE'.
(Other definitions for amicable that I've seen before include "Friendly, good-humoured" , "Friendly, peaceful" , "Friendly, reasonable" , "Friendly and sociable" , "Friendly, nonthreatening" .)