Gash tree, the outside being dead (8)
I believe the answer is:
lacerate
'gash tree' is the definition.
'lacerate' can be an answer for 'gash' (I've seen this in another clue). I am unsure of the 'tree' bit.
'outside being dead' is the wordplay.
'outside' is an insertion indicator (some letters go outside others).
'being' becomes 'cera' (I am not sure about this - if you are sure you should believe this answer much more).
'dead' becomes 'late' (similar in meaning).
'cera' put inside 'late' is 'LACERATE'.
'the' acts as a link.
Can you help me to learn more?
(Other definitions for lacerate that I've seen before include "Cut, leaving jagged edges" , "Deeply cut (flesh, skin)" , "Tear, rip (flesh or skin)" , "Tear roughly, cut jaggedly" , "Clear tea (anag.)" .)