Gap in seaside location penetrated by river (6)
I believe the answer is:
breach
'gap' is the definition.
(I know that gap can be written as breach)
'seaside location penetrated by river' is the wordplay.
I cannot really see how this works, but
'location' could be 'reach' (reach is a kind of location) and 'reach' is present in the answer.
A single letter 'b' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'in' is the link.
Can you help me to learn more?
(Other definitions for breach that I've seen before include "Gap made in a fortification" , "Split, break" , "Severance" , "Fissure" , "Infringement - gap" .)