Sentry found nothing right in hooligan (7)
I believe the answer is:
lookout
'sentry' is the definition.
(I know that sentry can be written as lookout)
'nothing right in hooligan' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'right' becomes 'ok' (both are affirmative answers).
'in' is an insertion indicator.
'hooligan' becomes 'lout' (both can mean an aggressive person).
'o'+'ok'='ook'
'ook' put within 'lout' is 'LOOKOUT'.
'found' acts as a link.
(Other definitions for lookout that I've seen before include "Outlook for sentry" , "Nightwatchman" , "A sentry or guard, must be watchful" , "Observer, sentinel" , "One keeping watch" .)