Companion left three miles behind company (9)
I believe the answer is:
colleague
'companion' is the definition.
(thesaurus)
'left three miles behind company' is the wordplay.
'left' becomes 'l' (common abbreviation).
'three miles' becomes 'league' (I've seen this before).
'behind' says to put letters next to each other.
'company' becomes 'co' (abbreviation).
'l'+'league'='lleague'
'lleague' put after 'co' is 'COLLEAGUE'.
(Other definitions for colleague that I've seen before include "Fellow worker" , "China, perhaps" , "Fellow-worker" , "fellow member of staff" , "Workmate" .)