Hold down core pressure to some extent (7)
I believe the answer is:
repress
'hold down' is the definition.
(thesaurus)
'core pressure to some extent' is the wordplay.
'to some extent' indicates a hidden word.
'REPRESS' is hidden within 'core pressure'.
(Other definitions for repress that I've seen before include "Put in irons, maybe" , "Subdue" , "Curb" , "Check" , "Keep in subjection" .)