A month back, foul up badly in it? That's irregular (9)
I believe the answer is:
guerrilla
'irregular' is the definition.
(synonyms)
'a month back foul up badly in it?' is the wordplay.
'a month' becomes 'Aug' (abbreviation for August).
'back' says the letters should be written in reverse.
'foul up' becomes 'err' (both can mean to blunder).
'badly' becomes 'ill' ('ill' can be adverbial as in 'ill-advised').
'in it?' means one lot of letters goes inside another.
'aug' backwards is 'gua'.
'err'+'ill'='errill'
'gua' going around 'errill' is 'GUERRILLA'.
'that's' is the link.
(Other definitions for guerrilla that I've seen before include "Irregular fighter" , "Irregular bandit" , "Combatant" , "Partisan" , "Member of an irregular armed force" .)