A sign outside ranch, home to old car marque (4,5)
I believe the answer is:
alfa romeo
'car marque' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'a sign outside ranch home to old' is the wordplay.
'sign' becomes 'leo' (star sign).
'outside' indicates putting letters inside (some letters go outside others).
'ranch' becomes 'farm' (I've seen this before).
'home to' means one lot of letters goes inside another (some letters house or home others).
'old' becomes 'o' (abbreviation).
'farm' placed around 'o' is 'farom'.
'leo' enclosing 'farom' is 'lfaromeo'.
'a'+'lfaromeo'='ALFA ROMEO'
Can you help me to learn more?
(Other definitions for alfa romeo that I've seen before include "sports car" , "Italian car" .)