'Horse's heart found in cooking pot' -- bold headline (8)
I believe the answer is:
streamer
'bold headline' is the definition.
(I've seen this before)
'horse's heart found in cooking pot' is the wordplay.
'horse's' becomes 'mare's' (mare is a kind of horse).
'heart' means to look at the middle letters.
'found in' indicates putting letters inside.
'cooking pot' becomes 'steamer' (pot for steaming things).
The middle letter of 'mares' is 'r'.
'r' going within 'steamer' is 'STREAMER'.
(Other definitions for streamer that I've seen before include "Ribbon for throwing" , "Ribbon - bad cold?" , "It blows in the wind" , "Long strip used for decoration or advertising" , "Item of bunting" .)