Sign by this motorway’s last for us, seeing that bank (5,9)
I believe the answer is:
yours sincerely
'sign by' is the definition.
The answer and definition are not the same part of speech.
'motorway's last for us seeing that bank' is the wordplay.
'motorway's last' becomes 'y' (I can't explain this - if you can you should give a lot more credence to this answer).
'for us' becomes 'ours' (I've seen this before).
'seeing that' becomes 'since' (both can mean 'when considering...').
'bank' becomes 'rely' (to bank on something is to rely on it).
'y'+'ours'+'since'+'rely'='YOURS SINCERELY'
'this' acts as a link.
Can you help me to learn more?
(Other definitions for yours sincerely that I've seen before include "so long" , "way of ending letter!" .)