Annoyed a number if overfed (6)

I believe the answer is:

miffed

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'annoyed' is the definition.
(I know that annoyed can be written as miffed)

'a number if overfed' is the wordplay.
'a number' becomes 'M' (M is an example).
'overfed' becomes 'fed' (I can't justify this - if you can you should give a lot more credence to this answer).
'm'+'if'+'fed'='MIFFED'

Can you help me to learn more?

(Other definitions for miffed that I've seen before include "Disgruntled" , "Annoyed, hacked off" , "Aggrieved" , "Informally offended, put out" .)

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

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