The number of oaks in place (5)
I believe the answer is:
seven
This clue could be a double definition.
'the number' is the first definition.
(seven is a kind of number)
'oaks in place' is the second definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'of' is the link.
Can you help me to learn more?
(Other definitions for seven that I've seen before include "- Against Thebes (Aeschylus)" , "Cube root of 343" , "Number of colours of the rainbow" , "Numeral" , "Theme of exotic dance" .)