Promotion of publicity on vehicle carrying mortar (11)
I believe the answer is:
advancement
'promotion' is the definition.
(synonyms)
'publicity on vehicle carrying mortar' is the wordplay.
'publicity' becomes 'ad' (short for advert).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'vehicle' becomes 'van' (type of vehicle).
'carrying' says to put letters next to each other (in a down clue, the bottom letters carry others).
'mortar' becomes 'cement' (I've seen this in other clues).
'ad'+'van'+'cement'='ADVANCEMENT'
'of' is the link.
(Other definitions for advancement that I've seen before include "Process of developing" , "Promotion in rank" , "getting on" , "headway" , "making progress" .)