Uniform black boards benefit duplicate (6)
I believe the answer is:
double
'duplicate' is the definition.
(I know that duplicate can be written as double)
'uniform black boards benefit' is the wordplay.
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'black' becomes 'b' (as in pencil classifications).
'boards' is an insertion indicator.
'benefit' becomes 'dole' (informal term for a state benefits).
'u'+'b'='ub'
'ub' placed inside 'dole' is 'DOUBLE'.
(Other definitions for double that I've seen before include "understudy" , "With two equal parts" , "Score of forty with one dart, perhaps?" , "Lookalike; twofold" , "Orwellian acceptance of conflicting principles" .)