Crossword Genius

Rotten PM no good after training (9)

Ross

I believe the answer is:

perishing

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'rotten pm no good after training' is the definition.
Although both the answer and definition are present participle verbs, I cannot understand how one could define the other.

'pm no good after training' is the wordplay.
'pm' becomes 'rishi' (I can't justify this - if you can you should believe this answer much more).
'no good' becomes 'ng' (abbreviation).
'after' says to put letters next to each other.
'training' becomes 'PE' (Physical Education).
'rishi'+'ng'='rishing'
'rishing' put after 'pe' is 'PERISHING'.

Can you help me to learn more?

(Other definitions for perishing that I've seen before include "Dying" , "going west" .)

I've seen this clue in The Independent.
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