Sudden change for American humorist into small red jumper (10)
I believe the answer is:
switcheroo
'sudden change for american' is the definition.
(informal US term for a sudden change)
'humorist into small red jumper' is the wordplay.
'humorist' becomes 'wit' (wit is a kind of humorist).
'into' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'red' becomes 'che' (Che Guevara the Marxist revolutionary).
'jumper' becomes 'roo' (I've seen this before**).
's'+'che'='sche'
'wit' put inside 'sche' is 'switche'.
'switche'+'roo'='SWITCHEROO'
(Another definition for switcheroo that I've seen is " Informal US variation".)