M Hulot’s creator backed in serious move (9)
I believe the answer is:
gravitate
'move' is the definition.
(gravitating is a kind of moving)
'm hulot's creator backed in serious' is the wordplay.
'm hulot's creator' becomes 'Tati' (Jacques Tati created Monsieur Hulot).
'backed' says the letters should be written backwards.
'in' means one lot of letters goes inside another.
'serious' becomes 'grave' (both can mean solemn).
'tati' in reverse letter order is 'itat'.
'itat' put inside 'grave' is 'GRAVITATE'.
(Other definitions for gravitate that I've seen before include "Move (towards) - be attracted to" , "Tend towards" , "Be drawn without conscious effort" , "drawn a certain way" , "sink" .)