Intrude, initially encountering natural resistance in bus (8)
I believe the answer is:
encroach
'intrude' is the definition.
(I've seen this before)
'initially encountering natural resistance in bus' is the wordplay.
'initially' indicates taking the first letters.
'resistance' becomes 'r' (electronics abbreviation).
'in' indicates putting letters inside.
'bus' becomes 'coach' (synonyms).
The initial letters of 'encountering natural' is 'en'.
'r' put into 'coach' is 'croach'.
'en'+'croach'='ENCROACH'
(Other definitions for encroach that I've seen before include "Make inroads" , "Enter upon another's property or rights" , "Gradually intrude (on person's area)" , "Advance beyond the usual limit" , "Go past a normal limit" .)