Of course, that lot on stag party can never say no
I believe the answer is:
yes men
'stag party can never say no' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'of course that lot' is the wordplay.
'of course' becomes 'yes' (I've seen this before).
'that lot' becomes 'men' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'yes'+'men'='YESMEN'
'on' is the link.
Can you help me to learn more?
(Other definitions for yes men that I've seen before include "They obey their bosses" , "Servile subordinates" , "crawlers?" , "lapdogs?" , "Unquestioning conformists" .)