Stop a hundred getting relaxation (5)
I believe the answer is:
cease
'stop' is the definition.
(I know that stop can be written as cease)
'a hundred getting relaxation' is the wordplay.
'a hundred' becomes 'C' (Roman numeral).
'getting' is a charade indicator (letters next to each other).
'relaxation' becomes 'ease' (synonyms).
'c'+'ease'='CEASE'
(Other definitions for cease that I've seen before include "Culminate" , "Finish, end" , "Discontinue" , "Stop, give over" , "Political group" .)