Undressed escort irrational round certain stars (7)
I believe the answer is:
scorpio
'certain stars' is the definition.
(constellation)
'undressed escort irrational round' is the wordplay.
'undressed' says to take the centre (the word removes the letters which clothe it).
'irrational' becomes 'pi' (an irrational number).
'round' becomes 'o' (round can mean a round object).
The centre of 'escort' is 'scor'.
'scor'+'pi'+'o'='SCORPIO'
(Other definitions for scorpio that I've seen before include "Zodiacal sign" , "Sign of autumn" , "A heavenly body" , "Oct-Nov zodiac sign" , "7 [SIGN OF THE ZODIAC]" .)