Facial expression - almost smile - by royal staff (7)
I believe the answer is:
grimace
'facial expression' is the definition.
(grimace is a kind of facial expression)
'almost smile by royal staff' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'smile' becomes 'grin' (synonyms).
'by' means one lot of letters go next to another.
'royal staff' becomes 'mace' (a mace is a ceremonial staff or stick. I am not sure about the 'royal' bit.).
'grin' with its final letter taken away is 'gri'.
'gri'+'mace'='GRIMACE'
Can you help me to learn more?
(Other definitions for grimace that I've seen before include "Dour" , "Disgusted expression" , "look disgusted" , "Distorted facial expression" , "Pained expression" .)