Puckish - that's the Spanish and French conclusion (5)
I believe the answer is:
elfin
'puckish' is the definition.
(I've seen this in another clue)
'the spanish and french conclusion' is the wordplay.
'the spanish' becomes 'el' ('the' in Spanish).
'and' means one lot of letters go next to another.
'french conclusion' becomes 'fin' (I can't justify this - if you can you should believe this answer much more).
'el'+'fin'='ELFIN'
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for elfin that I've seen before include "Small; impish" , "Like a little fairy" , "Petite and charming" , "Sprite-like" , "Delicate little" .)