It's most certain the others will follow us back (6)
I believe the answer is:
surest
'its most certain' is the definition.
I can't judge whether this defines the answer.
'others will follow us back' is the wordplay.
'others' becomes 'rest' (I've seen this in another clue).
'will follow' is a charade indicator (letters next to each other).
'back' shows that the letters should be reversed in order.
'us' backwards is 'su'.
'rest' put after 'su' is 'SUREST'.
'the' acts as a link.
Can you help me to learn more?
(Other definitions for surest that I've seen before include "Most certain or inevitable" , "Most convinced" , "Most certain and confident" , "With supreme confidence" , "Most secure" .)