Old flame nowt special? Tell me about it! (7)

I believe the answer is:

explain

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'nowt special? tell me about it' is the definition.
I can't judge whether this defines the answer.

'old flame' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'flame' becomes 'plain' (I can't justify this - if you can you should believe this answer much more).
'ex'+'plain'='EXPLAIN'

Can you help me to learn more?

(Other definitions for explain that I've seen before include "Resolve" , "Elucidate, make clear" , "Elucidate - define" , "Make (something) clear" , "Make understandable" .)

I've seen this clue in The Independent.

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