Donkey with tail docked has hint of mule, in a manner of speaking (5)
I believe the answer is:
idiom
'in a manner of speaking' is the definition.
(I've seen this before)
'donkey with tail docked has hint of mule' is the wordplay.
'donkey' becomes 'ii' (I am not sure about this - if you are sure you should believe this answer much more).
'with' is an insertion indicator.
'tail' says to take the final letters.
'has' is a charade indicator (letters next to each other).
'hint' indicates taking the first letters (I've seen 'hint of' mean this).
The final letter of 'docked' is 'd'.
The first letters of 'of mule' is 'om'.
'ii' placed around 'd' is 'idi'.
'idi'+'om'='IDIOM'
Can you help me to learn more?
(Other definitions for idiom that I've seen before include "characteristic use of language" , "Colloquial saying" , "Group of words in which the meaning may not be obvious" , "Form or variety of language" , "Figure of speech" .)