Between high and low tide — certainly, it's said (9)
I believe the answer is:
foreshore
'between high and low tide' is the definition.
(part of a shore between high and low water marks)
'certainly it's said' is the wordplay.
I cannot quite see how this works, but
'it's' could be 'es' ('e' can mean 'electronic' which is similar to 'IT') and 'es' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for foreshore that I've seen is " Area between high and low water marks".)