100 short, act on list of bequests as one wishes (2,4)
I believe the answer is:
at will
'as one wishes' is the definition.
(I've seen this in another clue)
'100 short act on list of bequests' is the wordplay.
'a00 short act' becomes 'at' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' says to put letters next to each other.
'list of bequests' becomes 'will' (list of bequests given after someone's death).
'at'+'will'='AT WILL'
Can you help me to learn more?
(Other definitions for at will that I've seen before include "when one wants" , "Fire .. .... - whenever you're ready" , "Whenever one wishes" , "Whenever you please, as you like" , "According to one's own inclination" .)