Such a river has its ups and downs (5)
I believe the answer is:
'such a river' is the definition.
I can't tell whether this definition defines the answer.
'its ups and downs' is the wordplay.
'its' becomes 'it' (I've seen this in other clues).
'ups' is a reversal indicator (I've seen 'up' mean this (in down clue: letters go upwards)).
'and' is a charade indicator (letters next to each other).
'downs' becomes 'dal' (I am not sure about this - if you are sure you should believe this answer much more).
'it' reversed gives 'ti'.
'has' acts as a link.
Can you help me to learn more?
(Other definitions for tidal that I've seen before include "subject to regular surges" , "Concerning the rise and fall of the sea" , "Pertaining to rise and fall of the sea" , "Relating to the flow of the sea" , "Relating to the rise and fall of the sea" .)