Credit a town with having no bridge at this vital point (4)
I believe the answer is:
crux
'this vital point' is the definition.
(I've seen this before)
'credit a town with having no bridge' is the wordplay.
I cannot really see how this works, but
'credit' could be 'cr' (abbreviation) and 'cr' is located in the answer.
This explanation may well be incorrect...
'at' acts as a link.
Can you help me to learn more?
(Other definitions for crux that I've seen before include "Baffling problem" , "Decisive point of an issue" , "Essential point in the matter or problem" , "Most important point at issue" , "Decisive, baffling, point" .)