Extremely eager dog restrained by gracious Queen's lead (7)
I believe the answer is:
mercury
'queen's lead' is the definition.
'Mercury' can be an answer for 'lead' (thesaurus). I am not certain of the 'queen's' bit.
'extremely eager dog restrained by gracious' is the wordplay.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'dog' becomes 'cur' (a cur is an aggressive or scruffy dog).
'restrained by' indicates putting letters inside.
'gracious' becomes 'my' (both are exclamations of surprise).
'eager' with its centre taken out is 'er'.
'er'+'cur'='ercur'
'ercur' placed inside 'my' is 'MERCURY'.
Can you help me to learn more?
(Other definitions for mercury that I've seen before include "Messenger of the Gods, or a chemical element" , "One associated with Queen" , "Planet - liquid metal" , "In Roman myth, the messenger of the gods" , "Messenger in Roman mythology" .)