Some fertiliser categorised over in the farmland units
I believe the answer is:
acres
'the farmland units' is the definition.
(I've seen this in another clue)
'some fertiliser categorised over' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'over' says the letters should be written in reverse.
'serca' is hidden within 'fertiliser categorised'.
'serca' written backwards gives 'ACRES'.
'in' is the link.
(Other definitions for acres that I've seen before include "may burden landed folk" , "fields?" , "Units of land measurement" , "Rival played" , "The Phoenix Park has 40 of them (5)" .)