Length being introduced to brown parsnip perhaps (10)
I believe the answer is:
umbellifer
'length' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'being introduced to brown parsnip perhaps' is the wordplay.
I cannot really see how this works, but
'being' could be 'lifer' (lifer is a kind of being) and 'lifer' is located in the answer.
The remaining letters 'umbel' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for umbellifer that I've seen before include "parsnip, perhaps?" , "Carrot" , "plant?" .)