Guardian piece about space (6)
I believe the answer is:
parent
'guardian' is the definition.
(thesaurus)
'piece about space' is the wordplay.
'piece' becomes 'part' (synonyms).
'about' indicates putting letters inside.
'space' becomes 'en' (in printing, a space as wide as a letter 'n').
'part' going around 'en' is 'PARENT'.
(Other definitions for parent that I've seen before include "Procreator" , "Uncle's sibling?" , "Author" , "Rear" , "Immediate ancestor" .)