There are waves when it turns (4)
I believe the answer is:
tide
'there are waves' is the definition.
I can't tell whether this defines the answer.
'when it turns' is the wordplay.
'when' becomes 'ed' (I am not sure about this - if you are sure you should believe this answer much more).
'turns' says the letters should be written backwards.
'ed'+'it'='edit'
'edit' reversed gives 'TIDE'.
Can you help me to learn more?
(Other definitions for tide that I've seen before include "Moon-driven sea movement" , "Current, ebb" , "Periodic rise and fall of the sea" , "Ebb and flow of water" , "Periodic water movement" .)