At home, guide becomes secondary (8)
I believe the answer is:
indirect
'secondary' is the definition.
(thesaurus)
'at home guide' is the wordplay.
'at home' becomes 'in' ('in' can be similar in meaning to 'at home').
'guide' becomes 'direct' ('direct' can be a synonym of 'guide').
'in'+'direct'='INDIRECT'
'becomes' acts as a link.
(Other definitions for indirect that I've seen before include "Not straighforward" , "In credit, not being straight" , "sort of speech" , "Oblique, not straight" , "In credit (anag.)" .)