Dietitian upset about slimmer's heart scare (10)
I believe the answer is:
intimidate
'scare' is the definition.
(intimidate can mean to scare)
'dietitian upset about slimmer's heart' is the wordplay.
'upset' indicates an anagram.
'about' is an insertion indicator.
'heart' indicates the central letters.
The centre of 'slimmer' is 'm'.
'dietitian' with letters rearranged gives 'intiidate'.
'intiidate' going around 'm' is 'INTIMIDATE'.
(Other definitions for intimidate that I've seen before include "Strike fear into" , "Scare" , "Threaten" , "Frighten" , "Bully" .)