Blockhead has defaced boarding cards (7)
I believe the answer is:
jackass
'blockhead' is the definition.
(I've seen this in another clue)
'has defaced boarding cards' is the wordplay.
'defaced' means to remove the first letter (the word with its face removed).
'boarding' means one lot of letters goes inside another.
'cards' becomes 'jacks' (jack is a kind of card).
'has' with its first letter taken away is 'as'.
'as' put inside 'jacks' is 'JACKASS'.
(Other definitions for jackass that I've seen before include "Male donkey; idiot" , "See 7" , "Charlie" , "One may lark about" , "Asinine person" .)