Upper-class learner beginning to bash into jolly tree (8)
I believe the answer is:
Here is my best explanation:
'tree' is the definition.
(mulberry is a kind of tree)
'upper-class learner beginning to bash into jolly' is the subsidiary indication.
'upper-class' becomes 'u' ('u' can be similar in meaning to 'upper-class').
'learner' becomes 'l'.
'beginning to' indicates taking the first letters.
'into' means one lot of letters goes inside another.
'jolly' becomes 'merry' ('merry' can be a synonym of 'jolly'**).
The first letter of 'bash' is 'b'.
'ulb' placed into 'merry' is 'mulberry'.
(Other definitions for mulberry that I've seen before include "So-called port substitute", "The tree on which the silkworm feeds", "Tree associated with silkworms", "the centre of attention in nursery circles?", "Fruit for silkworm", "Blackberry-like fruit", "plant", "Fruit producer", "Fruit tree; dark red" and "it should bear fruit".)