Shout about one climbing tree (5)
I believe the answer is:
lilac
'tree' is the definition.
(I've seen this before)
'shout about one climbing' is the wordplay.
'shout' becomes 'call' ('call' can be a synonym of 'shout').
'about' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'call' placed around 'i' is 'calil'.
'calil' back-to-front is 'LILAC'.
(Other definitions for lilac that I've seen before include "Light purple - shrub" , "Lavender-coloured" , "garden ornamental" , "Syringa vulgaris, light purple in colour" , "Pinkish purple - shrub" .)