Pluto has trousers but hers are lost. What a calamity! (10)
I believe the answer is:
disastrous
'trousers but hers are lost what a calamity' is the definition.
I can't tell whether this defines the answer.
'pluto' is the wordplay.
I cannot really see how this works, but
'pluto' could be 'dis' (another name for the mythical god) and 'dis' is present in the answer.
This explanation may well be incorrect...
'has' acts as a link.
Can you help me to learn more?
(Other definitions for disastrous that I've seen before include "Highly unsuccessful" , "Dire" , "Causing great damage" , "Catastrophic" .)