Fertiliser obtained from worker by river in Yorkshire (6)
I believe the answer is:
manure
'fertiliser' is the definition.
(I've seen this before)
'worker by river in yorkshire' is the wordplay.
'worker' becomes 'man' (man is a kind of worker).
'by' is a charade indicator (letters next to each other).
'river in yorkshire' becomes 'ure' (River Ure).
'man'+'ure'='MANURE'
'obtained from' acts as a link.
(Other definitions for manure that I've seen before include "should promote growth" , "Dung as fertiliser" , "Natural animal fertiliser" , "Animal excreta as fertiliser" , "Crap" .)