Queen of the Nile, informally (4)
I believe the answer is:
'queen of the nile informally' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cleo that I've seen before include "Jazz singer, ___ Laine" , "She ruled" , "Ms Laine - one of the 14 films" , "Woman" , "Dame Laine" .)