Next bird perched on side of nest (4)
I believe the answer is:
then
'next' is the definition.
(both can mean subsequently)
'bird perched on side of nest' is the wordplay.
I cannot quite see how this works, but
'bird' could be 'hen' (hen is a kind of bird) and 'hen' is found within the answer.
A single letter 't' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for then that I've seen before include "Subsequently - previously" , "At that time; next" , "soon?" , "Afterwards - as a result" , "Once upon a time" .)