Hoskins had undersized member? That's unfounded! (4)

I believe the answer is:

idle

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'that's unfounded' is the definition.
(synonyms)

'hoskins had undersized member?' is the wordplay.
'hoskins had' becomes 'I'd' (I had - where the crossword setter is 'Hoskins').
'undersized member?' becomes 'le' (I can't explain this - if you can you should believe this answer much more).
'id'+'le'='IDLE'

Can you help me to learn more?

(Other definitions for idle that I've seen before include "Not busy" , "Inert; lazy" , "out of a job" , "Not used to work" , "Frivolous" .)

I've seen this clue in The Independent.

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