Principal's mood something close to grave? (9)
I believe the answer is:
headstone
'something close to grave?' is the definition.
(graves normally have headstones)
'principal's mood' is the wordplay.
'principal's' becomes 'head's' ('head' can be a synonym of 'principal').
'mood' becomes 'tone' (tone is a kind of mood).
'heads'+'tone'='HEADSTONE'
(Other definitions for headstone that I've seen before include "Graveyard slab" , "Memorial on a grave" , "Its stands over a grave, duly inscribed" , "It is set up on a grave with inscriptions" , "It marks a grave" .)