Bear decapitated in destruction (4)
I believe the answer is:
ruin
'destruction' is the definition.
(ruin is a kind of destruction)
'bear decapitated' is the wordplay.
'bear' becomes 'bruin' (bruin is a kind of bear).
'decapitated' means to remove the first letter.
'bruin' with its first letter removed is 'RUIN'.
'in' acts as a link.
(Other definitions for ruin that I've seen before include "Total disaster" , "Destory or bankrupt" , "fall" , "dishonour" , "Dilapidated building" .)