Uniform son wears unadorned voluntarily (7)
I believe the answer is:
unasked
'voluntarily' is the definition.
(without being asked)
'uniform son wears unadorned' is the wordplay.
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'son' becomes 's' (genealogical abbreviation for son).
'wears' means one lot of letters goes inside another.
'unadorned' becomes 'naked' (I've seen this in another clue).
's' put into 'naked' is 'nasked'.
'u'+'nasked'='UNASKED'
(Other definitions for unasked that I've seen before include "(Done) voluntarily" , "Without being bidden" , "Without invitation" , "Gratuitous" , "Not invited" .)