Flexible prison sentence silly, ultimately (8)

I believe the answer is:

stretchy

'flexible' is the definition.
('stretchy' can be a synonym of 'flexible')

'prison sentence silly ultimately' is the wordplay.
'prison sentence' becomes 'stretch'.
'ultimately' suggests the final letters.
The final letter of 'silly' is 'y'.
'stretch'+'y'='STRETCHY'

(Other definitions for stretchy that I've seen before include "Flexible" , "Of variable length, perhaps" , "Elastic" .)

I've seen this clue in The Telegraph.

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