The woman at the heart of British Empire (3)
I believe the answer is:
she
'the woman' is the definition.
(I've seen this before)
'the heart of british empire' is the wordplay.
'the heart of' indicates a hidden word.
'SHE' is hidden in the letters of 'british empire'.
'at' is the link.
(Other definitions for she that I've seen before include "lady" , "Book" , "Rider's novel" , "Female pronoun" , "The one who must be obeyed" .)