A list of those on board? (9)
I believe the answer is:
directory
'a list' is the definition.
(thesaurus)
'of those on board?' is the wordplay.
I cannot quite see how this works, but
'of' could be 'o' and 'o' is found in the answer.
'on' could be 're' ('on' can be mean 'with reference to') and 're' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for directory that I've seen before include "Telephone information service" , "See 7" , "Reference book" , "list of contacts" , "Telephone book" .)