The most common and most likely answer to this crossword clue is the 7 letter word PORTAGE
I've seen this crossword clue in The Guardian
P
O
R
T
A
G
E
Buddy explains!

The straight part
We know that "conveyance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case conveyance = portage.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does PORTAGE mean?
PORTAGE noun- An act of carrying, especially the carrying of a boat overland between two waterways.
- The route used for such carrying.
- A charge made for carrying something.
- Carrying capacity; tonnage.
- The wages paid to a sailor when in port, or for a voyage.
- A porthole.
- To carry a boat overland