Flower power shortly facing the end of hostility (5)
I believe the answer is:
poppy
'flower' is the definition.
(I know that poppy is a type of flower)
'power shortly facing the end of hostility' is the wordplay.
'power shortly' becomes 'p' (symbol in physics; 'shortly' suggests a short form).
'facing' becomes 'opp' (abbreviation for opposite).
'the end of' says to take the final letters.
The final letter of 'hostility' is 'y'.
'p'+'opp'+'y'='POPPY'
(Other definitions for poppy that I've seen before include "catchy records like this" , "Red wild flower" , "Opium plant" , "Cornfield plant" , "Girl's name -- scarlet flower" .)