A sailor in the cast is naturally a good mimic (8)
I believe the answer is:
starling
'is naturally a good mimic' is the definition.
(I've seen this before)
'a sailor in the cast' is the wordplay.
'a sailor' becomes 'tar' (tar can mean a sailor).
'in' means one lot of letters goes inside another.
'the cast' becomes 'sling' (I've seen this in another clue).
'tar' put inside 'sling' is 'STARLING'.
(Other definitions for starling that I've seen before include "Stellar bird with blackish speckled feathers" , "Noisy bird with speckled feathers - somewhat stellar" , "Common garden bird" , "Slant rig for the strange bird" , "Dark, glossy bird" .)