Holds power in the group (6)
I believe the answer is:
clasps
'holds' is the definition.
('clasp' can be a synonym of 'hold')
'power in the group' is the wordplay.
'power' becomes 'p' (symbol in physics).
'in the' indicates putting letters inside.
'group' becomes 'class' (class is a kind of group).
'p' placed inside 'class' is 'CLASPS'.
(Other definitions for clasps that I've seen before include "Holds tightly" , "Holds or embraces closely" , "Embraces; fastenings" , "Necklace fasteners" , "Fasteners, as on necklaces perhaps" .)