Movement to adopt the first step (5)
I believe the answer is:
stair
'step' is the definition.
(a single step in a staircase)
'movement to adopt the first' is the wordplay.
'movement' becomes 'star' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to adopt' means one lot of letters goes inside another.
'the first' becomes 'i' (I in Roman numerals eg Elizabeth I).
'star' enclosing 'i' is 'STAIR'.
(Other definitions for stair that I've seen before include "Support on which to step" , "Component of flight" , "A step on the way up or down" , "One of a 15" , "something of a case" .)