First half evacuate returning to a hill hollow (4)
I believe the answer is:
cave
'hill hollow' is the definition.
'cave' can be an answer for 'hollow' (I've seen this in another clue). I'm not certain of the 'hill' bit.
'first half evacuate returning' is the wordplay.
'first half' indicates taking half.
'returning' shows that the letters should be reversed in order.
'evacuate' halved is 'evac'.
'evac' back-to-front is 'CAVE'.
'to a' is the link.
Can you help me to learn more?
(Other definitions for cave that I've seen before include "Beware" , "Large grotto" , "Warning - grotto" , "part of rocky coastline?" , "Primitive dwelling" .)