Calm a donkey with a fit of ague (7)
I believe the answer is:
assuage
'calm a donkey' is the definition.
'assuage' can be an answer for 'calm' (assuaging is a kind of calming). I am unsure of the 'a donkey' bit.
'with a fit of ague' is the wordplay.
'with' means one lot of letters goes inside another.
'a fit' becomes 'ass' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'of' indicates an anagram (the answer comes from or 'of' the letters).
'ague' anagrammed gives 'uage'.
'ass' put within 'uage' is 'ASSUAGE'.
Can you help me to learn more?
(Other definitions for assuage that I've seen before include "Allay, alleviate" , "allay hunger perhaps" , "'Make less severe, pain say (7)'" , "moderate" , "Soothe, make less severe" .)