Some maths makes my tree go funny (8)
I believe the answer is:
geometry
'some maths' is the definition.
(geometry is a kind of maths)
'my tree go funny' is the wordplay.
'funny' indicates anagramming the letters.
'my'+'tree'+'go'='mytreego'
'mytreego' anagrammed gives 'GEOMETRY'.
'makes' is the link.
(Other definitions for geometry that I've seen before include "branch of mathrmatics" , "Field of pure mathematics" , "Grey tome (anag.)" , "Branch of maths - ask Euclid" , "Euclid's science" .)