Sign accordingly giving envelope for my foreign letter (7)
I believe the answer is:
scorpio
'sign' is the definition.
(sign of the zodiac)
'accordingly giving envelope for my foreign letter' is the wordplay.
'accordingly' becomes 'so' (synonyms).
'giving envelope for' is an insertion indicator (the inserted letters are enveloped).
'my' becomes 'cor' (both are expressions of surprise).
'foreign letter' becomes 'pi' (letter of the Greek alphabet).
'cor'+'pi'='corpi'
'so' enclosing 'corpi' is 'SCORPIO'.
(Other definitions for scorpio that I've seen before include "A heavenly body" , "Ford model?" , "8th sign of the zodiac" , "Eighth sign of the Zodiac, kind of spider" , "Person born between 23 October and 22 November?" .)