Loamy ground underneath an irregularity (7)
I believe the answer is:
anomaly
'irregularity' is the definition.
(I've seen this in another clue)
'loamy ground underneath an' is the wordplay.
'ground' is an anagram indicator ('grind' the letters into a new form).
'underneath' says to put letters next to each other (in a down clue, some letters go under others).
'loamy' with letters rearranged gives 'omaly'.
'omaly' after 'an' is 'ANOMALY'.
(Other definitions for anomaly that I've seen before include "Irregular result" , "Inconsistency or deviation from general rule" , "Misfit - exception" , "Unexpected result" , "A deviation from normal" .)