Within a month, the rest developed expertise (7)
I believe the answer is:
mastery
'expertise' is the definition.
(I've seen this in another clue)
'within a month the rest developed' is the wordplay.
'within' means one lot of letters goes inside another.
'a month' becomes 'may' (May is an example).
'developed' indicates anagramming the letters.
'rest' is an anagram of 'ster'.
'may' enclosing 'ster' is 'MASTERY'.
(Other definitions for mastery that I've seen before include "Control - supreme skill" , "Acquirement" , "Supremacy" , "Expert skill" , "rule" .)