Heroine wants mutton: lamb’s heart sent back (4)
I believe the answer is:
emma
'heroine' is the definition.
(I've seen this before)
'mutton lamb's heart sent back' is the wordplay.
'mutton' becomes 'em' (synonyms).
'heart' means to look at the middle letters.
'sent back' says the letters should be written in reverse.
The middle letters of 'lamb' are 'am'.
'am' backwards is 'ma'.
'em'+'ma'='EMMA'
'wants' is the link.
(Other definitions for emma that I've seen before include "Lady - Hamilton, mistress of Nelson" , "It's a girl - ask me ma" , "Name of Nelson's Lady Hamilton" , "Austen character" , "classic work" .)