Dirty, therefore I left (4)

I believe the answer is:

soil

'dirty' is the definition.
('soil' can be a synonym of 'dirty')

'therefore i left' is the wordplay.
'therefore' becomes 'so'.
'left' becomes 'l' (common abbreviation).
'so'+'i'+'l'='SOIL'

(Other definitions for soil that I've seen before include "Sully with earth" , "refuse" , "Top layer of the surface of the earth" , "Earth; make dirty" , "Dirty - dirt" .)

I've seen this clue in The Telegraph.

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