Irregularity encountered probing army's new centre in Pontypool (9)
I believe the answer is:
asymmetry
'irregularity' is the definition.
(I've seen this before)
'encountered probing army's new centre in pontypool' is the wordplay.
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'probing' is an insertion indicator (probe can mean to poke or prod into).
'new' is an anagram indicator.
'centre in' means to look at the middle letters.
The central letter of 'pontypool' is 'y'.
'armys' is an anagram of 'asymr'.
'met' put within 'asymr' is 'asymmetr'.
'asymmetr'+'y'='ASYMMETRY'
(Other definitions for asymmetry that I've seen before include "Irregularity" , "with different faces" , "Lack of similarity between two parts or halves" , "displays a lack of regularity" .)