And possibly the wild grouse is a threat (9)
I believe the answer is:
dangerous
'is a threat' is the definition.
I can't judge whether this definition defines the answer.
'and possibly the wild grouse' is the wordplay.
'possibly' indicates an anagram.
'the wild' is an anagram indicator.
'and' with letters rearranged gives 'dan'.
'grouse' is an anagram of 'gerous'.
'dan'+'gerous'='DANGEROUS'
Can you help me to learn more?
(Other definitions for dangerous that I've seen before include "Unsafe, risky" , "High-risk" , "Not safe" , "Perilous, unsafe" , "posing risk" .)