Postholder’s speciality getting to the bottom of motorway trouble (7)
I believe the answer is:
mailbag
'postholder's' is the definition.
(something which holds post or mail)
'speciality getting to the bottom of motorway trouble' is the wordplay.
'speciality' becomes 'bag' (eg 'not my bag' means 'not my speciality').
'getting to the bottom of' says to put letters next to each other.
'motorway' becomes 'm'.
'trouble' becomes 'ail' (synonyms).
'm'+'ail'='mail'
'bag' after 'mail' is 'MAILBAG'.
(Other definitions for mailbag that I've seen before include "One sewn by convicts" , "Post pouch" , "Sack for carrying (American) letters" , "Correspondence" , "Postal sack" .)