Its happy ending comes as a relief (5)
I believe the answer is:
siege
'a relief' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Maybe you can see a link between them that I can't see?
'its happy ending comes as' is the wordplay.
I cannot quite understand how this works, but
'as' could be 'eg' (both can introduce an example) and 'eg' is present in the answer.
The remaining letters 'sie' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for siege that I've seen before include "Persistent attack" , "Isolating attack" , "Persistent attempt to gain" , "Operation to capture a fort, for example" , "Action that isolates and attacks" .)