Set forth from Dundee having been treated by P&O on board (9)
I believe the answer is:
expounded
'set forth' is the definition.
(both can mean outlined or presented)
'dundee having been treated by po on board' is the wordplay.
'having been treated' is an anagram indicator.
'by' becomes 'x' (2 by 4 is 2 x 4).
'on board' indicates putting letters inside (inserted letters are taken on board).
'x'+'po'='xpo'
'dundee' with letters rearranged gives 'eunded'.
'eunded' going around 'xpo' is 'EXPOUNDED'.
'from' acts as a link.
(Other definitions for expounded that I've seen before include "made clear" , "Explained" , "commented on" .)