Desmond marries but doesn't expect to be happy (8)
I believe the answer is:
despairs
'doesn't expect to be happy' is the definition.
I can't judge whether this defines the answer.
'desmond marries' is the wordplay.
'desmond' becomes 'des' (short for Desmond).
'marries' becomes 'pairs' (I can't explain this - if you can you should give a lot more credence to this answer).
'des'+'pairs'='DESPAIRS'
'but' is the link.
Can you help me to learn more?
(Other definitions for despairs that I've seen before include "Loses all hope" , "Abandons hope" .)