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' (I've seen this before).
'number fifty' becomes 'l' (Roman numeral).
'one' becomes 'ace' ('ace' can be a synonym of 'one').
'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" .)