At will, any mug can be prepared for soup (12)
I believe the answer is:
mulligatawny
'prepared for soup' is the definition.
'mulligatawny' can be an answer for 'soup' (mulligatawny is a kind of soup). I am not certain of the 'prepared for' bit.
'at will any mug can be' is the wordplay.
'can be' indicates an anagram.
'at'+'will'+'any'+'mug'='atwillanymug'
'atwillanymug' with letters rearranged gives 'MULLIGATAWNY'.
Can you help me to learn more?
(Other definitions for mulligatawny that I've seen before include "Spicy meat soup, originally from India" , "One type of soup" , "Indian soup" , "Curried soup" , "Curry flavoured soup" .)