Bird's mostly pert, naughtily holding ends of robe (6,6)
I believe the answer is:
stormy petrel
'bird's' is the definition.
(stormy petrel is a kind of bird)
'mostly pert naughtily holding ends of robe' is the wordplay.
'naughtily' indicates an anagram.
'holding' indicates putting letters inside.
'ends of' says to hollow out the word (remove centre letters).
'robe' with its centre removed is 're'.
'mostly'+'pert'='mostlypert'
'mostlypert' is an anagram of 'stormypetl'.
'stormypetl' placed around 're' is 'STORMY-PETREL'.
(Other definitions for stormy petrel that I've seen before include "one likely to cause trouble" , "Popular name for sea bird believed to presage bad weather" , "Mother Carey's chicken" , "Person who delights in controversy" , "seabird" .)