Head concealed in publicity material, shielding unknown secretive person (6)
I believe the answer is:
oyster
'secretive person' is the definition.
(I've seen this before)
'head concealed in publicity material shielding unknown' is the wordplay.
'head concealed in' means to remove the first letter.
'publicity material' becomes 'poster'.
'shielding' is an insertion indicator.
'unknown' becomes 'y' (algebra).
'poster' with its initial letter taken away is 'oster'.
'oster' placed around 'y' is 'OYSTER'.
(Other definitions for oyster that I've seen before include "Greyish pink" , "Content of stew" , "A pearl may grow in it" , "person unwilling to communicate" , "Bivalve - may be pearl carrier" .)