Every now and then, babies may roll over land (4)
I believe the answer is:
asia
'land' is the definition.
(Asia is an example)
'every now and then babies may roll over' is the wordplay.
'every now and then' means one should take alternating letters (letters selected at regular intervals).
'roll over' shows that the letters should be reversed in order (I've seen 'rolling over' mean this).
'babies'+'may'='babiesmay'
The alternate letters of 'babiesmay' are 'aisa'.
'aisa' back-to-front is 'ASIA'.
(Other definitions for asia that I've seen before include "group of countries" , "huge amount of ground" , "Large area" , "Continent bordering three oceans" , "Continental land mass" .)