Put up about one dozen signs (6)
I believe the answer is:
zodiac
'signs' is the definition.
(I've seen this before)
'put up about one dozen' is the wordplay.
'put up' is a reversal indicator.
'about' becomes 'ca' (circa).
'one' becomes 'i' (Roman numeral).
'dozen' becomes 'doz'.
'ca'+'i'+'doz'='caidoz'
'caidoz' reversed gives 'ZODIAC'.
(Other definitions for zodiac that I've seen before include "Belt of twelve signs" , "Astrologers' twelve signs" , "Diagram of astrological signs" , "Imaginary belt of heavenly signs" , "Aries, Taurus etc" .)