Stand up to soak sibling coming in (6)
I believe the answer is:
'stand up to' is the definition.
(I've seen this in another clue)
'soak sibling coming in' is the wordplay.
'soak' becomes 'ret' (ret can mean to soak in water).
'sibling' becomes 'sis' (short for sister).
'coming in' indicates putting letters inside.
'ret' placed around 'sis' is 'RESIST'.
(Other definitions for resist that I've seen before include "Take stand against, oppose" , "Forgo" , "Withstand or defy" , "Combat" , "Withstand a force" .)