Due to the clamp, being, indeed, unhappy (9)
I believe the answer is:
depressed
'unhappy' is the definition.
(I know this)
'due to the clamp being indeed' is the wordplay.
I cannot quite understand how this works, but
'clamp' could be 'press' (press is a kind of clamp) and 'press' is found within the answer.
The remaining letters 'deed' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for depressed that I've seen before include "Having been pushed down" , "25 [BLUE]" , "Sad or indented" , "Downhearted" , "Flattened downward" .)