Upper-class learner beginning to bash into jolly tree (8)
I believe the answer is:
mulberry
'tree' is the definition.
(mulberry is a kind of tree)
'upper-class learner beginning to bash into jolly' is the wordplay.
'upper-class' becomes 'u' ('u' can be similar in meaning to 'upper-class').
'learner' becomes 'l'.
'beginning to' suggests taking the first letters.
'into' is an insertion indicator.
'jolly' becomes 'merry' ('merry' can be a synonym of 'jolly'**).
The initial letter of 'bash' is 'b'.
'u'+'l'+'b'='ulb'
'ulb' placed within 'merry' is 'MULBERRY'.
(Other definitions for mulberry that I've seen before include "Fruit for silkworm" , "So-called port substitute" , "Fruit producer" , "Tree, its leaves fed to silkworms" , "essential to French invasion?" .)