Unsmiling Australian working in churchyard (11)
I believe the answer is:
gravedigger
'working in churchyard' is the definition.
(someone who might work in a churchyard)
'unsmiling australian' is the wordplay.
'unsmiling' becomes 'grave' (both can mean serious or solemn).
'australian' becomes 'digger' (term for an Australian soldier in World War I).
'grave'+'digger'='GRAVEDIGGER'
(Other definitions for gravedigger that I've seen before include "working in churchyard?" , "part in Hamlet" , "Cemetery worker" , "Sexton" , "One unearthing a plot" .)