Defence group expected to be extreme (5)
I believe the answer is:
undue
'be extreme' is the definition.
(I've seen this in another clue)
'defence group expected' is the wordplay.
'defence group' becomes 'un' (I can't justify this - if you can you should give a lot more credence to this answer).
'expected' becomes 'due' (similar in meaning).
'un'+'due'='UNDUE'
'to' is the link.
Can you help me to learn more?
(Other definitions for undue that I've seen before include "Excessive, unreasonable" , "More than appropriate" , "Extreme" , "Excessive - unwarranted" , "Excessively, unreasonably" .)