Jumper's state of anxiety about going upside down (7)
I believe the answer is:
sweater
'jumper's' is the definition.
(both can mean an item of clothing)
'state of anxiety about going upside down' is the wordplay.
'state of anxiety' becomes 'sweat' (as in getting into a sweat about something).
'about' becomes 're' ('re' means regarding or about).
'going upside down' says the letters should be written in reverse (in a down clue, letters go up).
're' written backwards gives 'er'.
'sweat'+'er'='SWEATER'
(Other definitions for sweater that I've seen before include "One visibly too hot" , "Heavy jersey" , "Warm garment" , "Jumper (7)" , "Guernsey" .)