Chance of bravery when losing power (4)
I believe the answer is:
luck
'chance' is the definition.
(I know that chance can be written as luck)
'bravery when losing power' is the wordplay.
'bravery' becomes 'pluck' (pluck is a kind of bravery).
'when losing' is a deletion indicator.
'power' becomes 'p' (symbol in physics).
'pluck' with 'p' taken out is 'LUCK'.
'of' acts as a link.
(Other definitions for luck that I've seen before include "'Fortune, fate (4)'" , "''The devil''s children has the devil's ....'" , "See 1" , "Chance, fate, fortune" , "Good or bad fortune" .)