Letter from internet company returned (3)
I believe the answer is:
psi
'letter' is the definition.
(I know that psi is a type of letter)
'internet company returned' is the wordplay.
'internet company' becomes 'ISP' (internet service provider).
'returned' shows that the letters should be reversed in order.
'isp' back-to-front is 'PSI'.
'from' acts as a link.
(Other definitions for psi that I've seen before include "Pounds per sq. in. (abbr.)" , "Greek letter; pounds per square inch (abbrev.)" , "letter from abroad" , "foreign letter" , "Pounds per square inch (abbr.); Greek letter" .)