Queen's Speech used by some to broaden one's horizon (7)
I believe the answer is:
seymour
'one's horizon' is the definition.
The definition suggests a singular noun which matches the answer.
'queen's speech used by some to broaden' is the wordplay.
I cannot quite see how this works, but
'queen's' could be 'our' (belonging to the monarch using the royal we) and 'our' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for seymour that I've seen before include "Jane -, mother of Edward VI" , "Jane - - , Edward VI's mother" , "Henry VIII's wife Jane" , "Jane --, queen" , "See 15" .)