Restless little devil getting a heart of flint in the pavilion (9)
I believe the answer is:
impatient
'restless' is the definition.
(similar in meaning)
'little devil getting a heart of flint in the pavilion' is the wordplay.
'little devil' becomes 'imp' (an imp is a little demon).
'getting' is a charade indicator (letters next to each other).
'heart of' means to look at the middle letters.
'in the' is an insertion indicator.
'pavilion' becomes 'tent' (I've seen this in another clue).
The middle letter of 'flint' is 'i'.
'i' going inside 'tent' is 'tient'.
'imp'+'a'+'tient'='IMPATIENT'
(Other definitions for impatient that I've seen before include "appeared anxious" , "Agitated" , "dying" , "restive" , "Restless; intolerant" .)