End period - natural, not new (9)
I believe the answer is:
terminate
'end' is the definition.
(synonyms)
'period natural not new' is the wordplay.
I cannot really see how this works, but
'period' could be 'term' (term is a kind of period) and 'term' is found in the answer.
'natural' could be 'nat' (academic abbreviation) and 'nat' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for terminate that I've seen before include "Kill off" , "Close" , "Finalise" , "Bring to an end" , "Conclude" .)