Male bottom becomes motionless (9)

I believe the answer is:

stagnates

I'm a little stuck... Click here to teach me more about this clue!

'becomes motionless' is the definition.
I can't judge whether this definition defines the answer.

'male bottom' is the wordplay.
'male' becomes 'stag' (a stag is a male deer).
'bottom' becomes 'nates' (I've seen this before).
'stag'+'nates'='STAGNATES'

Can you help me to learn more?

(Other definitions for stagnates that I've seen before include "becomes inactive" , "Festers" , "doesn't develop" .)

I've seen this clue in The Independent.

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