Flier in difficulty (4)
I believe the answer is:
knot
I believe this is a double definition.
'flier' is the first definition.
(knot is a kind of birdanimal)
'difficulty' is the second definition.
(I've seen this before)
'in' is the link.
(Other definitions for knot that I've seen before include "Tie (or what is put in one?)" , "A sheepshank or bowline" , "Ligature" , "that's sometimes in wood" , "Hard mass in tree trunk at branch joint" .)