Ghastly parliamentary staff must hold a Biro, oddly (7)
I believe the answer is:
macabre
'ghastly' is the definition.
(I know that ghastly can be written as macabre)
'parliamentary staff must hold a biro oddly' is the wordplay.
'parliamentary staff' becomes 'mace' (ceremonial staff in the Houses of Parliament).
'must hold' means one lot of letters goes inside another.
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
The alternating letters of 'biro' are 'br'.
'a'+'br'='abr'
'mace' placed around 'abr' is 'MACABRE'.
(Other definitions for macabre that I've seen before include "Brace ma, being gruesome" , "Dark and disturbing" , "Ghastly, gruesome" , "'Sinister, gruesome (7)'" , "Ghostly appearance" .)