One already mentioned mother's embraces (3)
I believe the answer is:
the
'one' is the definition.
(both can introduce a definite example of something)
'mentioned mother's embraces' is the wordplay.
'embraces' indicates a hidden word (I've seen 'embraced' mean this).
'THE' is hidden within 'mentioned mothers'.
'already' is the link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for the that I've seen before include "Born - - - - - - - - (carol)" , "Over - - moon" , "See 10 Across" , "Last words of story" , "15, 16 One given taste of own medicine" .)