Large emperor geese primarily imitate cuckoo sound (10)
I believe the answer is:
legitimate
'sound' is the definition.
(thesaurus)
'large emperor geese primarily imitate cuckoo' is the wordplay.
'primarily' suggests taking the first letters.
'cuckoo' indicates an anagram.
The initial letters of 'large emperor geese' is 'leg'.
'imitate' is an anagram of 'itimate'.
'leg'+'itimate'='LEGITIMATE'
(Other definitions for legitimate that I've seen before include "Within the rules" , "Genuine" , "authorised" , "Conforming to the law" , "It's admissible" .)