Gloomy number get behind (5)

I believe the answer is:

drear

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'gloomy' is the definition.
(I know that gloomy can be written as drear)

'number get behind' is the wordplay.
'number' becomes 'D' (D is an example).
'get behind' becomes 'rear' (I am not sure about this - if you are sure you should believe this answer much more).
'd'+'rear'='DREAR'

Can you help me to learn more?

(Other definitions for drear that I've seen before include "gloomy in poem" , "Morose" , "Poetic dull and uninteresting" , "Dismal, dull" , "Literarily gloomy" .)

I've seen this clue in The Irish Independent and The Globe and Mail.

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