Most angry if secret gets out (8)
I believe the answer is:
fiercest
'most angry' is the definition.
'if secret gets out' is the wordplay.
'gets out' is an anagram indicator (the letters get out of order).
'if'+'secret'='ifsecret'
'ifsecret' with letters rearranged gives 'FIERCEST'.
(Other definitions for fiercest that I've seen before include "Strongest" , "Most ferocious and savage" , "Cites ref as the most violent in temper" .)