Deceive in a business letter (3)
I believe the answer is:
con
'deceive' is the definition.
(I've seen this before)
'a business letter' is the wordplay.
'a business' becomes 'co' (short for company).
'letter' becomes 'n' ().
'co'+'n'='CON'
'in' is the link.
(Other definitions for con that I've seen before include "Defraud" , "Swindle or see" , "Direct steering; study" , "Not supporting" , "Study - swindle" .)