Monarch’s retinue about to receive English vegetable (9)
I believe the answer is:
courgette
'vegetable' is the definition.
(courgette is a kind of vegetable)
'monarch's retinue about to receive english' is the wordplay.
'monarch's retinue' becomes 'court' (I can't explain this - if you can you should give a lot more credence to this answer).
'about' is an insertion indicator.
'to receive' becomes 'get' (synonyms).
'english' becomes 'E' (abbreviation).
'court' enclosing 'get' is 'courgett'.
'courgett'+'e'='COURGETTE'
Can you help me to learn more?
(Other definitions for courgette that I've seen before include "Small green vegetable" , "One of the vegetables" , "this could supply kitchen" , "Zucchini is its American equivalent" , "Small marrow" .)