Murky offer to follow up furrow (6)
I believe the answer is:
turbid
'murky' is the definition.
(I know that murky can be written as turbid)
'offer to follow up furrow' is the wordplay.
'offer' becomes 'bid' (bid is a kind of offer).
'to follow' says to put letters next to each other (some letters go after or follow others).
'up' is a reversal indicator (in down clue: letters go upwards).
'furrow' becomes 'rut' (rut is a kind of furrow).
'rut' written backwards gives 'tur'.
'bid' after 'tur' is 'TURBID'.
(Other definitions for turbid that I've seen before include "Muddy, murky, cloudy" , "Dense, thick and cloudy" , "Disordered" , "(Of liquid) cloudy" , "Murky, clouded" .)