Attack that undermines public organisation (6)
I believe the answer is:
outfit
'organisation' is the definition.
(outfit is a kind of organization)
'attack that undermines public' is the wordplay.
'attack' becomes 'fit' (fit is a kind of attack).
'that undermines' means one lot of letters go next to another (in a down clue, some letters go underneath others).
'public' becomes 'out' (out can mean released to the public).
'fit' after 'out' is 'OUTFIT'.
(Other definitions for outfit that I've seen before include "Equip for an enterprise" , "Kit, rig-out" , "Costume" , "Organisation or set of clothing" , "Cohesive unit, eg a military company" .)