Implicate brother in climbing tree (7)
I believe the answer is:
embroil
'implicate' is the definition.
(I've seen this before)
'brother in climbing tree' is the wordplay.
'brother' becomes 'bro' (short for brother).
'in' means one lot of letters goes inside another.
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'tree' becomes 'lime' (type of tree).
'lime' backwards is 'emil'.
'bro' placed into 'emil' is 'EMBROIL'.
(Other definitions for embroil that I've seen before include "Involve (someone) in trouble" , "Involve (a person) in some sort of trouble" , "Incriminate" , "Bring into confusion" , "Drag in - bole rim (anag)" .)