Trembling like mother of cygnets (5)
I believe the answer is:
aspen
'trembling' is the definition.
(I've seen this before)
'like mother of cygnets' is the wordplay.
'like' becomes 'as'.
'mother of cygnets' becomes 'pen' (I can't justify this - if you can you should give a lot more credence to this answer).
'as'+'pen'='ASPEN'
Can you help me to learn more?
(Other definitions for aspen that I've seen before include "Trembling poplar tree" , "winter resort" , "somewhere in Colorado" , "A poplar with shivering leaves" , "Tmorous" .)