One dying for a cause in irregular army crossing centre of Khartoum (6)
I believe the answer is:
martyr
'one dying for a cause' is the definition.
(I've seen this before)
'irregular army crossing centre of khartoum' is the wordplay.
'irregular' indicates an anagram.
'crossing' means one lot of letters goes inside another.
'centre of' says to take the centre.
The central letters of 'khartoum' are 'rt'.
'army' with letters rearranged gives 'mayr'.
'mayr' placed around 'rt' is 'MARTYR'.
'in' is the link.
(Other definitions for martyr that I've seen before include "One dying for their faith" , "Victim for a cause" , "One exaggerating suffering" , "One suffering for beliefs" , "Person dying for their faith" .)