Diana duck in freezing, showing stupidity (6)
I believe the answer is:
idiocy
'stupidity' is the definition.
(I know that idiocy is a type of backwardness)
'diana duck in freezing' is the wordplay.
'diana' becomes 'Di' (Di is short for Diana).
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'in' means one lot of letters goes inside another.
'freezing' becomes 'icy' (synonyms).
'di'+'o'='dio'
'dio' going into 'icy' is 'IDIOCY'.
'showing' is the link.
(Other definitions for idiocy that I've seen before include "Extreme silliness" , "Great folly" , "Imbecility" , "Extreme foolishness" , "Stupid act" .)