Longest of three years in penthouse, debauched (10)
I believe the answer is:
hypotenuse
'longest of three' is the definition.
(longest line in a triangle)
'years in penthouse debauched' is the wordplay.
'years' becomes 'y'.
'in' is an insertion indicator.
'debauched' indicates anagramming the letters (I've seen this in other clues).
'penthouse' with letters rearranged gives 'hpotenuse'.
'y' going inside 'hpotenuse' is 'HYPOTENUSE'.
(Other definitions for hypotenuse that I've seen before include "Right-angled triangle's longest side" , "Longest side of right-angled triangle" , "Long side of right-angled triangle" , "Subject of 1 across's theorem" , "Side opposite right angle" .)