Bird in skylark, no throstle! (4)
I believe the answer is:
knot
'bird' is the definition.
(knot is a kind of bird)
'in skylark no throstle' is the wordplay.
'in' says the answer is hidden in the clue.
'KNOT' is hidden in the letters of 'skylark no throstle'.
(Other definitions for knot that I've seen before include "It may be tied" , "Nautical measure" , "Fastener - bird - unit of speed at sea" , "Shorebird's" , "Hard irregularity in timber - interlacing of cord" .)