Going mad circling bottom of oak tree (6)

'tree' is the definition.
(ginkgo is a kind of tree)

'going mad circling bottom of oak' is the wordplay.
'mad' indicates an anagram.
'circling' is an insertion indicator (circling can mean going around or containing).
'bottom of' says to take the final letters.
The final letter of 'oak' is 'k'.
'going' anagrammed gives 'gingo'.
'gingo' placed around 'k' is 'GINKGO'.