Actress opens mammoth novel (4,8)
I believe the answer is:
emma thompson
'actress' is the definition.
'opens mammoth novel' is the wordplay.
'novel' is an anagram indicator (letters in a new or novel order).
'opens'+'mammoth'='opensmammoth'
'opensmammoth' anagrammed gives 'EMMA THOMPSON'.
(Other definitions for emma thompson that I've seen before include "Best Actress Oscar winner 1992, for "Howard's End"" , "Theatrical 12/17/19/28 [CANTABRIGIAN]" .)