Chemical engineers needing sales rep (7)
I believe the answer is:
reagent
'chemical' is the definition.
(thesaurus)
'engineers needing sales rep' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'needing' says to put letters next to each other.
'sales rep' becomes 'agent' (I've seen this before).
're'+'agent'='REAGENT'
(Other definitions for reagent that I've seen before include "Chemical with known properties" , "Active chemical" , "Chemically-active substance" , "Reacting substance" , "Chemical substance used in tests" .)