Criminal is vain with no occupation (8)
I believe the answer is:
invasion
'occupation' is the definition.
(I've seen this before)
'criminal is vain with no' is the wordplay.
'criminal' indicates an anagram.
'with' says to put letters next to each other.
'is'+'vain'='isvain'
'isvain' put after 'no' is 'noisvain'.
'noisvain' is an anagram of 'INVASION'.
(Other definitions for invasion that I've seen before include "Impetuous; skin eruption" , "Incursive attack" , "Incursion, entry for conquest" , "Offensive" , "Unwelcome intrusion" .)