Had group that's armed Isis held every now and then (9)
I believe the answer is:
possessed
'had group' is the definition.
'possessed' can be an answer for 'had' (possessing is a kind of having). I am unsure of the 'group' bit.
'armed isis held every now and then' is the wordplay.
'armed' becomes 'posse' (I can't justify this - if you can you should give a lot more credence to this answer).
'every now and then' means one should take alternating letters (letters selected at regular intervals).
'isis'+'held'='isisheld'
The alternate letters of 'isisheld' are 'ssed'.
'posse'+'ssed'='POSSESSED'
'that's' is the link.
Can you help me to learn more?
(Other definitions for possessed that I've seen before include "Taken over and owned - by the devil?" , "controlled by inner demons?" , "in need of bell, book and candle" , "being mad?" , "Demented" .)