Don't say it's obligatory (10)
I believe the answer is:
imperative
This clue is a double definition.
'don't say' is the first definition.
('don't' is an example of an imperative)
'obligatory' is the second definition.
(I've seen this before)
'it's' acts as a link.
(Other definitions for imperative that I've seen before include "Urgently necessary" , "mood" , "Essential" , "Crucial" , "Requiring attention" .)