He leaves the layer at that time (4)
I believe the answer is:
then
'at that time' is the definition.
('then' can be a synonym of 'at that time')
'he leaves the layer' is the wordplay.
'leaves' is a deletion indicator.
'layer' becomes 'hen' (I've seen this before**).
'the' with 'he' removed is 't'.
't'+'hen'='THEN'
(Other definitions for then that I've seen before include "Afterwards - as a result" , "Consequently" , "At that time; next" , "soon?" , "Subsequently - previously" .)