Cross party set up as extremists ignored lines (8)
I believe the answer is:
traverse
'cross' is the definition.
(I know that cross can be written as traverse)
'party set up as extremists ignored lines' is the wordplay.
'party' becomes 'tr' (I can't explain this - if you can you should give a lot more credence to this answer).
'set up' becomes 'installed' (I've seen this in another clue).
'as extremists ignored' says to take the centre (letters on the extremes are removed).
'lines' becomes 'verse' (lines of poetry).
The centre of 'installed' is 'a'.
'tr'+'a'+'verse'='TRAVERSE'
(Other definitions for traverse that I've seen before include "get across" , "Go across" , "cover" , "Travel across or pass over" , "Move across" .)