Weakish Shadow roughing up barmen (8)
I believe the answer is:
penumbra
'weakish shadow' is the definition.
'penumbra' can be an answer for 'shadow' (penumbra is a kind of shadow). I'm not sure about the 'weakish' bit.
'roughing up barmen' is the wordplay.
'roughing' indicates an anagram (I've seen 'rough' mean this (a rough or broken ordering of the letters)).
'up'+'barmen'='upbarmen'
'upbarmen' with letters rearranged gives 'PENUMBRA'.
Can you help me to learn more?
(Other definitions for penumbra that I've seen before include "Outer shadow round object" , "Imperfect shadow" , "Ring around sunspot" , "Outer region of shadow" , "Shady area" .)