Blue with radical serviceman (11)
I believe the answer is:
ultramarine
'blue' is the definition.
(I know that ultramarine is a type of blue)
'with radical serviceman' is the wordplay.
'with' is a charade indicator (letters next to each other).
'radical' becomes 'ultra' (synonyms).
'serviceman' becomes 'marine' (marine is a kind of serviceman).
'ultra'+'marine' is 'ULTRAMARINE'.
(Other definitions for ultramarine that I've seen before include "See a colour" , "one of the colours" , "Tone" , "Deep blue pigment" , "Brilliant deep blue" .)