Bob with a BA in a special tree (6)
I believe the answer is:
baobab
'special tree' is the definition.
'baobab' can be an answer for 'tree' (baobab is a kind of tree). I'm not certain of the 'special' bit.
'bob with a ba' is the wordplay.
'with' means one lot of letters goes inside another.
'ba' becomes 'AB' (synonyms).
'bob' enclosing 'a' is 'baob'.
'baob'+'ab'='BAOBAB'
'in a' is the link.
Can you help me to learn more?
(Other definitions for baobab that I've seen before include "African fruit tree" , "Thick-trunked tree" , "Gigantic tropical tree with very thick trunk" , "African or Australian tree with very thick trunk" , "Exotic tree" .)