Rebels have one that's superior, just in case (5)
I believe the answer is:
cause
'rebels have' is the definition.
'cause' can be an answer for 'have' (thesaurus). I am not certain of the 'rebels' bit.
'one that's superior just in case' is the wordplay.
'one that's superior just' becomes 'u' (I can't explain this - if you can you should give a lot more credence to this answer).
'in' is an insertion indicator.
'u' placed inside 'case' is 'CAUSE'.
Can you help me to learn more?
(Other definitions for cause that I've seen before include "It brings about an effect" , "Producer of an effect" , "Give rise to" , "Produce (an effect)" , "Make happen; something deserving support" .)