Monarch in step becomes barren (7)
I believe the answer is:
sterile
'barren' is the definition.
(I've seen this in another clue)
'monarch in step' is the wordplay.
'monarch' becomes 'er' (abbreviation for Elizabeth Regina, British Queen).
'in' indicates putting letters inside.
'step' becomes 'stile'.
'er' put into 'stile' is 'STERILE'.
'becomes' acts as a link.
(Other definitions for sterile that I've seen before include "Unable to have children" , "Free from living micro-organisms, absolutely clean" , "like a mule" , "Not producing offspring" , "Unable to produce young" .)