Split first part of the tree up (4)
I believe the answer is:
rift
'split' is the definition.
(I've seen this before)
'first part of the tree up' is the wordplay.
'first part of' indicates taking the first letters.
'tree' becomes 'fir' (fir is a kind of tree).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
The first letter of 'the' is 't'.
't'+'fir'='tfir'
'tfir' in reverse letter order is 'RIFT'.
(Other definitions for rift that I've seen before include "Fissure; split" , "Breach, rupture" , "Schism (in the lute?)" , "Breach or crevasse" , "Serious falling out" .)