Ruler collapsed on march (7)
I believe the answer is:
monarch
'ruler' is the definition.
(monarch is a kind of ruler)
'collapsed on march' is the wordplay.
'collapsed' indicates anagramming the letters.
'on'+'march'='onmarch'
'onmarch' with letters rearranged gives 'MONARCH'.
(Other definitions for monarch that I've seen before include "King or butterfly" , "Crowned head" , "Not a subject" , "Sovereign; butterfly" , "Head of State" .)