Antique George left in trailer (3-3)
I believe the answer is:
age-old
'antique' is the definition.
(I know that antique can be written as age-old)
'george left in trailer' is the wordplay.
'george' becomes 'geo'.
'left' becomes 'l' (common abbreviation).
'in' means one lot of letters goes inside another.
'trailer' becomes 'ad' (short for advert).
'geo'+'l'='geol'
'geol' inserted into 'ad' is 'AGE-OLD'.
(Other definitions for age-old that I've seen before include "Belonging to times long ago" , "Having been around for many years" , "Having existed for a very long time" , "Hallowed by antiquity" , "Extremely ancient" .)