No good blocking second astrological symbol as personal emblem (8)
I believe the answer is:
monogram
'personal emblem' is the definition.
'no good blocking second astrological symbol' is the wordplay.
'good' becomes 'g' (abbreviation).
'blocking' is an insertion indicator.
'second' becomes 'mo' (both are short periods of time).
'astrological symbol' becomes 'ram' (sign of the zodiac).
'no'+'g'='nog'
'mo'+'ram'='moram'
'nog' going into 'moram' is 'MONOGRAM'.
'as' acts as a link.
(Other definitions for monogram that I've seen before include "Motif formed of initials" , "Personal logo" , "Symbolic initials" , "Motif designed around letters" , "union of characters" .)