Heading for parliament in secret, maltreated royal staff (7)
I believe the answer is:
sceptre
'royal staff' is the definition.
(I've seen this before)
'heading for parliament in secret maltreated' is the wordplay.
'heading for' suggests taking the first letters.
'in' is an insertion indicator.
'maltreated' indicates anagramming the letters.
The first letter of 'parliament' is 'p'.
'secret' anagrammed gives 'scetre'.
'p' going within 'scetre' is 'SCEPTRE'.
(Other definitions for sceptre that I've seen before include "Royal baton" , "Part of the royal regalia" , "Queen's staff" , "Rod of office" , "Monarch's ceremonial staff" .)