Flock of geese hide round the top of the elm (5)
I believe the answer is:
skein
'flock of geese' is the definition.
(I've seen this in another clue)
'hide round the top of the elm' is the wordplay.
'hide' becomes 'skin' (a hide is an animal skin).
'round' is an insertion indicator.
'the top of' suggests taking the first letters.
The first letter of 'elm' is 'e'.
'skin' enclosing 'e' is 'SKEIN'.
(Other definitions for skein that I've seen before include "Loose bundle of yarn" , "Birds flying" , "Hank" , "A number of birds" , "Loosely-coiled bundle of wool" .)