From list of bequests, one at a time sons cut out by choice (9)
I believe the answer is:
willingly
'from list of bequests' is the definition.
I can't judge whether this definition defines the answer.
'one at a time sons cut out by choice' is the wordplay.
'one at a time' becomes 'singly' (eg not in pairs).
'sons' becomes 's'.
'cut out' indicates named letters should be taken away.
'by' says to put letters next to each other.
'choice' becomes 'will' (someone's will is their decision or choice).
'singly' with 's' taken away is 'ingly'.
'ingly' after 'will' is 'WILLINGLY'.
(Other definitions for willingly that I've seen before include "Nothing loth" , "Without demur" , "without hesitation" .)