Ruler against stopping progress (7)
I believe the answer is:
monarch
'ruler' is the definition.
(monarch is a kind of ruler)
'against stopping progress' is the wordplay.
'against' becomes 'on' (on can mean by or up against).
'stopping' is an insertion indicator (inserted letters stop or plug a gap).
'progress' becomes 'march' (march is a kind of progress).
'on' inserted inside 'march' is 'MONARCH'.
(Other definitions for monarch that I've seen before include "Head of State" , "Butterfly - ruler" , "Eg, king, queen, emperor" , "Sovereign; butterfly" , "Butterfly (emperor, say?)" .)