In animus, tranquillity is essential (4)
I believe the answer is:
must
'essential' is the definition.
(I know that must is a type of essential)
'in animus tranquillity is' is the wordplay.
'in' indicates the answer is hidden within the clue.
'MUST' is hidden in the letters of 'animus tranquillity is'.
(Other definitions for must that I've seen before include "Prerequisite" , "The mould is compelled" , "Grape juice ready for fermentation" , "Is obliged to" , "Unfermented wine" .)