Wading bird in flock, no tail (4)
I believe the answer is:
knot
'wading bird' is the definition.
(knot is a kind of wading bird)
'in flock no tail' is the wordplay.
'in' indicates a hidden word.
'KNOT' is hidden in the letters of 'flock no tail'.
(Other definitions for knot that I've seen before include "Difficulty" , "complex problem" , "Tight interlacing of cord, string etc" , "Shore bird, tangle" , "Small group (of people); tied rope" .)