Decline beginning to reappear in party power (4)
I believe the answer is:
drop
'decline' is the definition.
(I know that decline is a more specific form of the action drop)
'beginning to reappear in party power' is the wordplay.
'beginning to' suggests taking the first letters.
'in' is an insertion indicator.
'party' becomes 'do' (do is a kind of party).
'power' becomes 'p' (symbol in physics).
The first letter of 'reappear' is 'r'.
'r' inserted inside 'do' is 'dro'.
'dro'+'p'='DROP'
(Other definitions for drop that I've seen before include "See 1" , "Tiny bit of liquid" , "Dispense with - sink" , "Fall down with liquid globule" , "shed" .)