Bill Nighy's opening door (7)
I believe the answer is:
postern
'door' is the definition.
(I've seen this before)
'bill nighy's opening' is the wordplay.
'bill' becomes 'poster' (synonyms).
'opening' indicates taking the first letters.
The first letter of 'nighy' is 'n'.
'poster'+'n'='POSTERN'
(Other definitions for postern that I've seen before include "private entrance" , "where tradesmen enter?" , "a way in the back" , "Back door eg of castle" , "Small gate in the rear of a fort or castle" .)