River rising in parallel intervals (4)
I believe the answer is:
nile
'river' is the definition.
(I know that Nile is a river)
'rising in parallel intervals' is the wordplay.
'rising' says the letters should be written in reverse (in a down clue, letters go up).
'in' indicates a hidden word.
'elin' is hidden within 'parallel intervals'.
'elin' in reverse letter order is 'NILE'.
(Other definitions for nile that I've seen before include "long-distance runner in Africa" , "Take a line to the river" , "River of eastern Africa" , "Egyptian river" , "Long river" .)