Monstrous error on pitch by leaderless Irish (6)
I believe the answer is:
ogrish
'monstrous' is the definition.
(I've seen this in another clue)
'error on pitch by leaderless irish' is the wordplay.
'error on pitch' becomes 'og' (football abbreviation for own goal).
'by' is a charade indicator (letters next to each other).
'leaderless' means to remove the first letter.
'irish' with its initial letter removed is 'rish'.
'og'+'rish'='OGRISH'
(Other definitions for ogrish that I've seen before include "Demonic; brutish" , "Cruel and terrifying" .)