Bird: that female nesting in most important tree (10)
I believe the answer is:
kingfisher
'bird' is the definition.
(kingfisher is a kind of bird)
'female nesting in most important tree' is the wordplay.
'female' becomes 'she' ('she' refers to a woman).
'nesting in' is an insertion indicator.
'most important' becomes 'king' (I can't explain this - if you can you should believe this answer much more).
'tree' becomes 'fir' (fir tree is a kind of tree).
'king'+'fir'='kingfir'
'she' going within 'kingfir' is 'KINGFISHER'.
'that' is the link.
Can you help me to learn more?
(Other definitions for kingfisher that I've seen before include "Colourful river bird" , "Brightly-coloured bird" , "flyer" , "Colourful riverside bird" , "Creature" .)