Writer’s union leading motion to support area of uncertainty (8)
I believe the answer is:
penumbra
'area of uncertainty' is the definition.
(a vague or grey area)
'writer's union leading motion to support' is the wordplay.
'writer' becomes 'pen' (I've seen this before).
'union' becomes 'u' (common abbreviation as in 'Trades Union Congress').
'leading' says to take the initial letters.
'to support' becomes 'bra' (a support for the breasts).
The first letter of 'motion' is 'm'.
'pen'+'u'+'m'+'bra'='PENUMBRA'
(Other definitions for penumbra that I've seen before include "Shady area" , "Outer shadow round object" , "Outer region of shadow" , "Area of half shade" , "Imperfect shadow" .)