To carry a number of passengers, they’re ludicrous (6)
I believe the answer is:
farces
'passengers they're ludicrous' is the definition.
The definition suggests an adverb but the answer is not.
'to carry a number' is the wordplay.
'to' becomes 'fares' (I can't justify this - if you can you should believe this answer much more).
'carry' is an insertion indicator.
'a number' becomes 'c' (C is an example).
'fares' going around 'c' is 'FARCES'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for farces that I've seen before include "Comedies with ridiculously improbable situations" , "Comedies of extravagant humour" .)