Back cunning Irish to defend against cross (8)
I believe the answer is:
traverse
'cross' is the definition.
('traverse' can be a synonym of 'cross')
'back cunning irish to defend against' is the wordplay.
'back cunning' becomes 'tra' (I am not sure about this - if you are sure you should believe this answer much more).
'irish' becomes 'erse' (I've seen this before).
'to defend' means one lot of letters goes inside another.
'against' becomes 'v' (abbreviation for 'versus').
'tra'+'erse'='traerse'
'traerse' enclosing 'v' is 'TRAVERSE'.
Can you help me to learn more?
(Other definitions for traverse that I've seen before include "cover" , "In mountaineering, a steep slope to be crossed from side to side" , "Extend or bridge across" , "Go across" , "Ski diagonally across (a slope)" .)