Fill in for salesman number fifty one (7)
I believe the answer is:
replace
'fill in for' is the definition.
(thesaurus)
'salesman number fifty one' is the wordplay.
'salesman' becomes 'rep' (sales rep).
'number fifty' becomes 'L' (Roman numeral).
'one' becomes 'ace' (ace has a value of one in some card games).
'rep'+'l'+'ace'='REPLACE'
(Other definitions for replace that I've seen before include "Put back in original position" , "Substitute for (someone)" , "Put back or substitute" , "Put back where you found it" , "succeed" .)