Trouper's safe, heading off aboard wagon (7)
I believe the answer is:
veteran
'trouper's' is the definition.
(thesaurus)
'safe heading off aboard wagon' is the wordplay.
'safe' becomes 'peter' (I've seen this before).
'heading off' means to remove the first letter.
'aboard' is an insertion indicator.
'wagon' becomes 'van'.
'peter' with its first letter removed is 'eter'.
'eter' put within 'van' is 'VETERAN'.
(Other definitions for veteran that I've seen before include "Describing cars built before 1919, especially before 1905" , "Old-stager" , "Describing a pre-1905 car (or pre-1919 at a pinch?)" , "One long experienced" , "One with lengthy experience in service" .)