Work up on coach carriage (7)
I believe the answer is:
postage
'carriage' is the definition.
(thesaurus)
'work up on coach' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'up' is a reversal indicator (in down clue: letters go upwards).
'on' says to put letters next to each other.
'coach' becomes 'stage' (stage is a kind of coach).
'op' in reverse letter order is 'po'.
'po'+'stage'='POSTAGE'
(Other definitions for postage that I've seen before include "Charge for 23 Acrosses" , "Amount paid to mail" , "It could be first-class" , "Mailing cost" , "Cost of sending letter" .)