Ruminants destroying good crop (4)
I believe the answer is:
oats
'crop' is the definition.
(type of plant)
'ruminants destroying good' is the wordplay.
'ruminants' becomes 'goats' (goats are ruminants).
'destroying' indicates named letters should be taken away.
'good' becomes 'g' (abbreviation).
'goats' with 'g' removed is 'OATS'.
(Other definitions for oats that I've seen before include "Porridge ingredients" , "Porridge ingredient" , "Sow wild ones in disreputable youth" , "Hardy cereals eaten by horses" , "Grain for porridge" .)