Notice one at the centre queued for so long (5)
I believe the answer is:
adieu
'so long' is the definition.
('adieu' can be a synonym of 'so long')
'notice one at the centre queued' is the wordplay.
'notice' becomes 'ad'.
'one' becomes 'i' (Roman numeral).
'at' says to put letters next to each other.
'the centre' indicates the central letters.
The centre of 'queued' is 'eu'.
'ad'+'i'+'eu'='ADIEU'
'for' is the link.
(Other definitions for adieu that I've seen before include "the last word in Paris" , "Farewel" , "I'm away (in France?)" , "Goodbye to the French" , "So long abroad" .)