Some scared lads up a tree (5)
I believe the answer is:
alder
'a tree' is the definition.
(alder is a kind of tree)
'some scared lads up' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'redla' is hidden in the letters of 'scared lads'.
'redla' written backwards gives 'ALDER'.
(Other definitions for alder that I've seen before include "Tree or shrub whose wood is resistant to underwater rot" , "Catkin-bearing tree" , "Tree with catkins" , "One [TYPE OF TREE]" , "Tree akin to the birch" .)