Shadow barmen up for replacement (8)
I believe the answer is:
penumbra
'shadow' is the definition.
(penumbra is a kind of shadow)
'barmen up for replacement' is the wordplay.
'for replacement' is an anagram indicator (I've seen this in other clues).
'barmen'+'up'='barmenup'
'barmenup' with letters rearranged gives 'PENUMBRA'.
(Other definitions for penumbra that I've seen before include "Ring around sunspot" , "Imperfect shadow" , "Outer shadow round object" , "Shady area" , "Outer region of shadow" .)