### Clue: Comparatively sensitive whistleblower's upset about heartless intimidation (5)
### Answer: FINER 
 ### Explanation: Definition: Comparatively sensitive
 **Reversal of**(…'s upset, in a down clue) REF(short "referee", a match umpire who uses a whistle to signal) **containing**(about) "intimidation" **minus all its inner letters**(heartless …).