Periodically spying on policy to do with grammar (9)
I believe the answer is:
syntactic
'do with grammar' is the definition.
I can't tell whether this defines the answer.
'periodically spying on policy' is the wordplay.
'periodically' means one should take alternating letters (letters must be selected 'every so often').
'on policy' becomes 'tactic' (I can't justify this - if you can you should give a lot more credence to this answer).
The alternating letters of 'spying' are 'syn'.
'syn'+'tactic'='SYNTACTIC'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for syntactic that I've seen before include "following the rules of grammar" , "dealing with arrangement of sentences" .)