The number of trees associated with Covent Garden (4,4)
I believe the answer is:
nine elms
'covent garden' is the definition.
I know nothing about this answer so I cannot tell whether this works.
'the number of trees associated with' is the wordplay.
'the number of' becomes 'nine' (I can't explain this - if you can you should give a lot more credence to this answer).
'trees' becomes 'elms' (elm is a kind of tree).
'associated with' means one lot of letters go next to another.
'nine'+'elms'='NINE ELMS'
Can you help me to learn more?
(Another definition for nine elms that I've seen is " part of Wandsworth - with 25d".)