Though priestly, he did hide some of the rules (5)
I believe the answer is:
druid
'though priestly he did hide some of the rules' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'though priestly he did hide' is the wordplay.
'though priestly he' becomes 'ru' (I can't justify this - if you can you should give a lot more credence to this answer).
'hide' means one lot of letters goes inside another.
'ru' going inside 'did' is 'DRUID'.
Can you help me to learn more?
(Other definitions for druid that I've seen before include "old magician?" , "Ancient Celtic priest" , "Ancient Celt priest" , "Member of Welsh Gorsedd; ancient priest" , "Old Celtic priest" .)