Some signs unknown company holds help back (6)
I believe the answer is:
zodiac
'some signs' is the definition.
(I've seen this before)
'unknown company holds help back' is the wordplay.
'unknown' becomes 'z' (represents an unknown value in algebra).
'company' becomes 'co' (abbreviation).
'holds' is an insertion indicator.
'help' becomes 'aid' (synonyms).
'back' says the letters should be written backwards.
'co' enclosing 'aid' is 'caido'.
'caido' reversed gives 'odiac'.
'z'+'odiac'='ZODIAC'
(Other definitions for zodiac that I've seen before include "Celestial region" , "astrological study" , "Band of sky much used by astrologists" , "It's divided into 12 signs" , "Imaginary belt in the heavens" .)