Light infantry in degrading defeat (10)
I believe the answer is:
demolition
'defeat' is the definition.
(I've seen this before)
'light infantry in degrading' is the wordplay.
I cannot really see how this works, but
'light' could be 'lit' ('lit' can be similar in meaning to 'light') and 'lit' is found within the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for demolition that I've seen before include "Licking" , "destructive action" , "explosion" , "ruin" , "defeat" .)