May your beginning be powerful (6)
I believe the answer is:
mighty
'be powerful' is the definition.
(similar in meaning)
'may your beginning' is the wordplay.
'may' becomes 'might' ('may' and 'might' are similar in meaning).
'beginning' says to take the initial letters.
The first letter of 'your' is 'y'.
'might'+'y'='MIGHTY'
(Other definitions for mighty that I've seen before include "Powerful - exalted" , "Arrogant people are often described as high and . . . . . ." , "Strong, powerful" , "Large" , "Very powerful/strong" .)