Press release initially upset EMI chief (5)
I believe the answer is:
prime
'chief' is the definition.
(thesaurus)
'press release initially upset emi' is the wordplay.
'initially' suggests taking the first letters.
'upset' says the letters should be written in reverse.
The first letters of 'press release' is 'pr'.
'emi' in reverse letter order is 'ime'.
'pr'+'ime'='PRIME'
(Other definitions for prime that I've seen before include "golden age?" , "Foremost" , "Monastic service; best quality" , "Maybe eleven" , "Chief or most important, like a minister" .)