One helping a relative to cross last section of bypass (8)
'section of bypass' is the definition.
I know nothing about this answer so I can't judge whether this works.
'one helping a relative to cross last' is the wordplay.
'one' becomes 'a'.
'helping a relative' becomes 'sister' (I can't justify this - if you can you should give a lot more credence to this answer).
'to' means one lot of letters go next to another (I've seen this in other clues).
'last' indicates one should take the final letters.
The last letter of 'cross' is 's'.
'sister' put after 's' is 'ssister'.
Can you help me to learn more?
(Another definition for assister that I've seen is "Someone who helps".)