Notice Her majesty about to choose one to take on? (7)
I believe the answer is:
'one to take on?' is the definition.
I can't judge whether this definition defines the answer.
'notice her majesty about to choose' is the wordplay.
'notice' becomes 'ad' (short for advert).
'her majesty' becomes 'ER' (Elizabeth Regina).
'about' means one lot of letters go next to another.
'to choose' becomes 'opt' (both can mean to pick something).
'er' after 'opt' is 'opter'.
Can you help me to learn more?
(Other definitions for adopter that I've seen before include "A redtop (anag.)", "person to accept another's child?", "who extends a family welcome?", "Person taking legal responsibility for a child not biologically theirs", "One may take issue".)