Notes in Brussels primarily imply lethargy (5)
I believe the answer is:
ennui
'lethargy' is the definition.
(I've seen this in another clue)
'notes in brussels primarily imply' is the wordplay.
'notes' becomes 'nn' (abbreviation for 'note' twice).
'in' means one lot of letters goes inside another.
'brussels' becomes 'EU' (European Parliament is based in Brussels).
'primarily' says to take the initial letters.
The first letter of 'imply' is 'i'.
'nn' going inside 'eu' is 'ennu'.
'ennu'+'i'='ENNUI'
(Other definitions for ennui that I've seen before include "Weary feeling" , "Low interest rate" , "Boredom, dullness" , "Feeling of weariness or boredom" , "Boredom, tedium" .)