Split trousers initially after climbing tree (4)
I believe the answer is:
rift
'split' is the definition.
(I've seen this in other clues)
'trousers initially after climbing tree' is the wordplay.
'initially' says to take the initial letters.
'after' is a charade indicator (letters next to each other).
'climbing' is a reversal indicator (in a down clue, letters climb up).
'tree' becomes 'fir' (fir tree is a kind of tree).
The initial letter of 'trousers' is 't'.
'fir' in reverse letter order is 'rif'.
't' put after 'rif' is 'RIFT'.
(Other definitions for rift that I've seen before include "Breach - falling-out" , "Fissure in something" , "Break in relations; type of valley" , "Crack, split, break" , "Breach; crack" .)