Irreverence of soldiers in Africa sentry regularly dismissed (7)
I believe the answer is:
impiety
'irreverence' is the definition.
(impiety is a kind of irreverence)
'soldiers in africa sentry regularly dismissed' is the wordplay.
'soldiers in africa' becomes 'impi' (an impi is a group of Zulu soldiers).
'regularly dismissed' means one should take alternating letters (letters dismissed at regular intervals).
The alternating letters of 'sentry' are 'ety'.
'impi'+'ety'='IMPIETY'
'of' is the link.
(Other definitions for impiety that I've seen before include "Irreverence, profanity" , "sacrilege" , "Lack of reverence" , "godlessness" , "Irreligion" .)