Lawless American agent held in custody at first (8)
I believe the answer is:
anarchic
'lawless' is the definition.
(synonyms)
'american agent held in custody at first' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'agent' becomes 'narc' (narcotics officer).
'at first' indicates taking the first letters.
The first letters of 'held in custody' is 'hic'.
'a'+'narc'+'hic'='ANARCHIC'
(Other definitions for anarchic that I've seen before include "Disorderly" , "disregarding the law" , "Uncontrolled or lawless" , "lacking proper rule" , "With no ruling principles to give order" .)