Favouring, at the outset, plenty of trees (6)
I believe the answer is:
forest
'of trees' is the definition.
The definition suggests a singular noun which matches the answer.
'favouring at the outset plenty' is the wordplay.
'favouring' becomes 'for' (to be 'for' something is to be in favour of it).
'at' says to put letters next to each other.
'the outset plenty' becomes 'est' (I can't justify this - if you can you should give a lot more credence to this answer).
'for'+'est'='FOREST'
Can you help me to learn more?
(Other definitions for forest that I've seen before include "large area covered with trees" , "eg Sherwood" , "Large area of trees and undergrowth" , "Best Actor Oscar winner 2006, for 'The Last King of Scotland'" , "See 15" .)