Disappointment in the French town about duke (3-4)
I believe the answer is:
let-down
'disappointment' is the definition.
(I've seen this before)
'the french town about duke' is the wordplay.
'the french' becomes 'le' ('the' in French).
'about' means one lot of letters goes inside another.
'duke' becomes 'd' (abbreviation).
'town' enclosing 'd' is 'tdown'.
'le'+'tdown'='LET-DOWN'
'in' is the link.
(Other definitions for let-down that I've seen before include "It's disappointing" , "Anticlimax" , "Disappointment" .)