Unknown helper wearing firm round belt (6)
I believe the answer is:
zodiac
'belt' is the definition.
(I have seen 'Astrological belt ' mean 'zodiac' so perhaps 'belt' could also mean 'zodiac')
'unknown helper wearing firm round' is the wordplay.
'unknown' becomes 'z' (represents an unknown value in algebra).
'helper' becomes 'aid' (something which helps).
'wearing' means one lot of letters goes inside another.
'firm' becomes 'co' (a firm is a company).
'round' says the letters should be written in reverse.
'aid' inserted within 'co' is 'caido'.
'caido' backwards is 'odiac'.
'z'+'odiac'='ZODIAC'
(Other definitions for zodiac that I've seen before include "Imaginary belt in the heavens" , "Astrologers' set of signs" , "Imaginary belt of heavenly signs" , "Its signs are in the heavens" , "Astrologers' twelve signs" .)