Linnets disturbed by good little bird (8)
I believe the answer is:
nestling
'little bird' is the definition.
(I've seen this before)
'linnets disturbed by good' is the wordplay.
'disturbed' indicates anagramming the letters (the order of letters is disturbed).
'by' is a charade indicator (letters next to each other).
'good' becomes 'g' (abbreviation).
'linnets' with letters rearranged gives 'nestlin'.
'nestlin'+'g'='NESTLING'
(Other definitions for nestling that I've seen before include "Young bird not yet fledged" , "in a huddle" , "Baby bird" , "Sitting snugly" , "Little bird" .)