Struggle continues with external support (6)
I believe the answer is:
resist
'struggle' is the definition.
(I know that resist is a type of defend)
'continues with external support' is the wordplay.
'continues with' becomes 'is' (I am not sure about this - if you are sure you should believe this answer much more).
'external' means one lot of letters goes inside another (I've seen this in other clues).
'support' becomes 'rest' (e.g. a shoulder rest is a support for shoulder).
'is' put inside 'rest' is 'RESIST'.
Can you help me to learn more?
(Other definitions for resist that I've seen before include "Bar" , "Put up opposition to sister" , "Check; withstand" , "Stand up to, fight against" , "Put up opposition, defy" .)