Ruler performing in a month (7)
I believe the answer is:
monarch
'ruler' is the definition.
(monarch is a kind of ruler)
'performing in a month' is the wordplay.
'performing' becomes 'on' (I've seen this in other clues).
'in' is an insertion indicator.
'a month' becomes 'march' (March is an example**).
'on' going within 'march' is 'MONARCH'.
(Other definitions for monarch that I've seen before include "one over realm?" , "Eg, king, queen, emperor" , "Butterfly (emperor, say?)" , "Stag in Landseer picture" , "Crowned head" .)