Power source so many determined to hold up (6)
I believe the answer is:
dynamo
'power source' is the definition.
(I've seen this before)
'so many determined to hold up' is the wordplay.
'to hold' indicates a hidden word (I've seen 'holding' mean this).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'omanyd' is hidden in the letters of 'so many determined'.
'omanyd' in reverse letter order is 'DYNAMO'.
(Other definitions for dynamo that I've seen before include "Device producing electrical power" , "Generator of electricity (eg, on bicycle)" , "Electric current producer" , "Energy converter - hard worker" , "Electric current generator" .)