Leotard's divertingly worn by male artist (3,6)
'artist' is the definition.
(old master is a kind of artist)
'leotard's divertingly worn by male' is the wordplay.
'divertingly' is an anagram indicator (letters diverted to different places).
'worn by' means one lot of letters goes inside another (I've seen this in other clues).
'male' becomes 'm' (common abbreviation).
'leotards' is an anagram of 'oldaster'.
'oldaster' placed around 'm' is 'OLD MASTER'.
(Other definitions for old master that I've seen before include "great work", "Famous painting", "A great European painter - a painting by one", "great artist", "Great painting of former times".)