Act is fool's last during diplomatic mission (11)
I believe the answer is:
legislation
'diplomatic mission' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'act is fool's last during' is the wordplay.
'act' becomes 'legation' (legation is a kind of act).
'last' says to take the final letters.
'during' means one lot of letters goes inside another.
The last letter of 'fool' is 'l'.
'is'+'l'='isl'
'legation' placed around 'isl' is 'LEGISLATION'.
Can you help me to learn more?
(Other definitions for legislation that I've seen before include "Enacting of laws" , "Passing of bills" , "Lawmaking" , "Group of laws" , "Statute law" .)