As one chooses a material (2,4)
I believe the answer is:
at will
'as one chooses' is the definition.
('at will' can be a synonym of 'as one chooses')
'a material' is the wordplay.
'material' becomes 'twill' (twill is a kind of material**).
'a'+'twill'='AT WILL'
(Other definitions for at will that I've seen before include "Fire .. .... - whenever you're ready" , "As one chooses" , "Whenever one pleases, own choice" , "when one wants" , "According to one's own inclination" .)