Postie occasionally delivers foreign letter (3)
I believe the answer is:
psi
'foreign letter' is the definition.
(I've seen this before)
'postie occasionally' is the wordplay.
'occasionally' means one should take alternating letters.
The alternate letters of 'postie' are 'PSI'.
'delivers' is the link.
(Other definitions for psi that I've seen before include "Pounds per sq. in. (abbr.)" , "Pounds per square inch (abbr.); Greek letter" , "Greek letter; pressure unit" , "Twenty-third letter of Greek alphabet" , "Greek character" .)