National freedom fighters quit at regular intervals (5)
I believe the answer is:
iraqi
'national' is the definition.
(I've seen this before)
'freedom fighters quit at regular intervals' is the wordplay.
'freedom fighters' becomes 'ira' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'at regular intervals' means one should take alternating letters.
The alternating letters of 'quit' are 'qi'.
'ira'+'qi'='IRAQI'
Can you help me to learn more?
(Other definitions for iraqi that I've seen before include "native of troubled land" , "Baghdadi, perhaps" , "From Basra, possibly" , "Person from Baghdad?" , "Middle Easterner" .)