Catastrophe in the sack, upset with lace pants (7)
I believe the answer is:
debacle
'catastrophe' is the definition.
(a debacle is a catastrophe or disaster)
'sack upset with lace pants' is the wordplay.
'sack' becomes 'bed' ('in the sack' means 'in bed').
'upset' shows that the letters should be reversed in order.
'with' means one lot of letters go next to another.
'pants' indicates anagramming the letters (UK informal term for 'nonsense' or 'rubbish').
'bed' reversed gives 'deb'.
'lace' anagrammed gives 'acle'.
'deb'+'acle'='DEBACLE'
'in the' is the link.
(Other definitions for debacle that I've seen before include "Total disorder or defeat" , "great failure" , "Rout; fiasco" , "Sudden and ignominious failure" , "Sudden disastrous collapse or defeat" .)