The star is upset by what anonymous letters may contain (7)
I believe the answer is:
threats
'anonymous letters may contain' is the definition.
I can't judge whether this definition defines the answer.
'the star is upset by' is the wordplay.
'is upset by' indicates anagramming the letters.
'the'+'star'='thestar'
'thestar' with letters rearranged gives 'THREATS'.
'what' is the link.
Can you help me to learn more?
(Other definitions for threats that I've seen before include "Sources of danger -- shatter (anag)" , "Shatter (anag) - menaces" , "Terrifying demands" , "Statements of intentions to harm" , "Expressions of intention to harm" .)