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' says the answer is hidden in the clue (I've seen 'embraced' mean this).
'THE' can be found hidden inside 'mentioned mothers'.
'already' is the link.
I am not very happy about this link. It may be part of 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" .)