Calm about decline in fortune (11)
I believe the answer is:
serendipity
'fortune' is the definition.
(thesaurus)
'calm about decline' is the wordplay.
'calm' becomes 'serenity' (serenity is a kind of calm**).
'about' means one lot of letters goes inside another.
'decline' becomes 'dip' (dipping is a kind of declining).
'serenity' placed around 'dip' is 'SERENDIPITY'.
'in' acts as a link.
(Other definitions for serendipity that I've seen before include "Beneficial happenstance" , "Fortunate chance" , "Lucky happenstance" , "happy coincidence" , "The accidental discovery of good fortune" .)