Extent to which tides can flow? (8)
I believe the answer is:
distance
'extent' is the definition.
(I have seen 'The extent' mean 'distance' so perhaps 'extent' could also mean 'distance')
'tides can flow?' is the wordplay.
'flow?' indicates anagramming the letters (to flow can mean to move around).
'tides'+'can'='tidescan'
'tidescan' anagrammed gives 'DISTANCE'.
'to which' acts as a link.
(Other definitions for distance that I've seen before include "The gap between two places" , "Range" , "Length between points - remoteness" , "See 3" , "league, for example" .)