Small tree presents an insurmountable problem when not on-site (8)

I believe the answer is:

laburnum

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'small tree' is the definition.
'laburnum' can be an answer for 'tree' (type of European tree). I'm not sure about the 'small' bit.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for laburnum that I've seen before include "Poisonous flowering shrub - run album (anag)" , "Shrub known as golden chain" , "Flowering tree" , "Poisonous plant" , "Tree or shrub with bright yellow flowers" .)

I've seen this clue in the Sydney Morning Herald.

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