Learner in Saab bumped tree (5)
I believe the answer is:
balsa
'tree' is the definition.
(balsa is a kind of tree)
'learner in saab bumped' is the wordplay.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'in' means one lot of letters goes inside another.
'bumped' indicates anagramming the letters.
'saab' is an anagram of 'basa'.
'l' going within 'basa' is 'BALSA'.
(Other definitions for balsa that I've seen before include "Strong, light wood" , "Very light wood in a slab" , "Central American forest tree used to make floats, rafts, models etc" , "Very light timber" , "Something light" .)