Birds keep moving south to the other side (4)
I believe the answer is:
aves
'birds keep moving' is the definition.
'aves' can be an answer for 'birds' (I've seen this before). I am not sure about the 'keep moving' bit.
'south to the other side' is the wordplay.
I cannot quite see how this works, but
'south' could be 's' (abbreviation) and 's' is found in the answer.
The remaining letters 'ave' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for aves that I've seen is " Reverential salutations: class of vertebrates which comprises birds".)