Stop those ultimately involved in legal action (5)
I believe the answer is:
cease
'stop' is the definition.
(synonyms)
'those ultimately involved in legal action' is the wordplay.
'ultimately' indicates one should take the final letters.
'involved in' indicates putting letters inside.
'legal action' becomes 'case' (synonyms).
The last letter of 'those' is 'e'.
'e' inserted inside 'case' is 'CEASE'.
(Other definitions for cease that I've seen before include "Stop, give over" , "Discontinue" , "Political group" , "Desist" , "Stop, terminate" .)