Mobilised clan crossing border, one that's illegal (8)
I believe the answer is:
criminal
'illegal' is the definition.
(similar in meaning)
'mobilised clan crossing border one' is the wordplay.
'mobilised' indicates an anagram (I've seen 'mobilize' mean this (I've seen this in other clues)).
'crossing' indicates putting letters inside (to cross can mean to go over or around).
'border' becomes 'rim' (both can mean an edge).
'one' becomes 'i' (Roman numeral).
'clan' with letters rearranged gives 'cnal'.
'rim'+'i'='rimi'
'cnal' going around 'rimi' is 'CRIMINAL'.
'that's' acts as a link.
(Other definitions for criminal that I've seen before include "Deplorable, shocking" , "An habitual breaker of the law" , "Being sought by police" , "Felon, lawbreaker" , "Person who breaks the law for a living" .)