Second American admitting irregular unit is insubordinate (8)
I believe the answer is:
mutinous
'insubordinate' is the definition.
(similar in meaning)
'second american admitting irregular unit' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'american' becomes 'us' (abbreviation for United States).
'admitting' means one lot of letters goes inside another (admit can mean to welcome in or accept).
'irregular' is an anagram indicator.
'mo'+'us'='mous'
'unit' anagrammed gives 'utin'.
'mous' enclosing 'utin' is 'MUTINOUS'.
'is' acts as a link.
(Other definitions for mutinous that I've seen before include "Rebellious like the Bounty's crew" , "Tending to rebel" , "Unsubmissive" , "Insubordinate" , "Defiantly disobedient" .)