Tree cracking up under the surface (9)
I believe the answer is:
macadamia
'tree' is the definition.
(macadamia is a kind of tree)
'cracking up under the surface' is the wordplay.
'cracking' becomes 'AI' (resembles Am, informal term for excellent).
'up' is a reversal indicator (in down clue: letters go upwards).
'under' says to put letters next to each other (below, in a down clue).
'the surface' becomes 'macadam' (macadam is a kind of surface).
'ai' back-to-front is 'ia'.
'ia' after 'macadam' is 'MACADAMIA'.
(Other definitions for macadamia that I've seen before include "Am I a cad, ma? (anag) - Australian nut tree" , "Queensland nut" , "Edible nut" , "(Of drug) causing dependency" , "Nut-bearing tree" .)