Generator country set up after disheartening day (6)
I believe the answer is:
dynamo
'generator' is the definition.
(dynamo is a kind of generator)
'country set up after disheartening day' is the wordplay.
'country' becomes 'oman' (Oman is an example**).
'set up' is a reversal indicator.
'after' means one lot of letters go next to another.
'disheartening' suggests removing the centre.
'day' with its middle taken out is 'dy'.
'oman' written backwards gives 'namo'.
'namo' after 'dy' is 'DYNAMO'.
(Other definitions for dynamo that I've seen before include "Electricity generator: all-action type" , "Tirelessly energetic type" , "Electricity generator or energetic person" , "Generator of electricity (eg, on bicycle)" , "Energetic person (colloq.)" .)