Becomes liable to Independent regarding attack (9)
I believe the answer is:
incursion
'attack' is the definition.
(I know that incursion is a type of attack)
'becomes liable to independent regarding' is the wordplay.
'becomes liable to' becomes 'incurs' ('incur' can be a synonym of 'become liable to').
'independent' becomes 'i'.
'regarding' becomes 'on' (on or regarding a particular topic).
'incurs'+'i'+'on'='INCURSION'
(Other definitions for incursion that I've seen before include "Hostile raid" , "Attack made into enemy territory" , "Foray" , "Sudden attack" .)