He, barring deviation, becomes a herald (9)
I believe the answer is:
harbinger
'a herald' is the definition.
(I know that herald can be written as harbinger)
'he barring deviation' is the wordplay.
'deviation' is an anagram indicator.
'he'+'barring'='hebarring'
'hebarring' with letters rearranged gives 'HARBINGER'.
'becomes' acts as a link.
(Other definitions for harbinger that I've seen before include "One giving advance warning" , "One announcing approach" , "Herald - forerunner" , "Early sign" , "It indicates the future" .)