Stop small company taking fifty on (5)
I believe the answer is:
colon
'stop' is the definition.
(thesaurus)
'small company taking fifty on' is the wordplay.
'small company' becomes 'co'.
'taking' says to put letters next to each other.
'fifty' becomes 'l' (roman numeral).
'co'+'l'+'on'='COLON'
(Other definitions for colon that I've seen before include "cut short dependency?" , "almost the end of another" , "City at the Caribbean entrance to the Panama Canal" , "Punctuation mark; gut" , ": - bit of the gut" .)