Filled with spirit, steed periodically follows groups of lawmen (9)
I believe the answer is:
possessed
'filled with spirit' is the definition.
(eg possessed by a demon)
'steed periodically follows groups of lawmen' is the wordplay.
'periodically' means one should take alternating letters (letters must be selected 'every so often').
'follows' says to put letters next to each other.
'groups of lawmen' becomes 'posses' (posse can mean a group of lawmen).
The alternate letters of 'steed' are 'sed'.
'sed' put after 'posses' is 'POSSESSED'.
(Other definitions for possessed that I've seen before include "Owned - by evil spirit?" , "Bore" , "Owned or controlled - by the devil?" , "controlled by inner demons?" , "being under a spell" .)