Clue not regularly seen as sign (3)
I believe the answer is:
leo
'sign' is the definition.
(star sign)
'clue not regularly seen' is the wordplay.
'regularly seen' means one should take alternating letters.
'clue'+'not'='cluenot'
The alternate letters of 'cluenot' are 'LEO'.
'as' is the link.
(Other definitions for leo that I've seen before include "Lion sign of the Zodiac" , "Zodiacal lion" , "Singer-songwriter Sayer, b. 1949" , "Pope" , "Star sign of one born on August 1" .)