Tranquillity Base involved in step (5)
I believe the answer is:
peace
'tranquillity' is the definition.
(I know that peace is a type of tranquillity)
'base involved in step' is the wordplay.
'base' becomes 'e' (Euler's number - base of natural logarithms).
'involved in' indicates putting letters inside.
'step' becomes 'pace' (pace is a kind of step).
'e' inserted within 'pace' is 'PEACE'.
(Other definitions for peace that I've seen before include "A Christmas wish" , "Calm, tranquility" , "Calmness" , "See 1" , "State of freedom from war" .)