Keep aunt and sis in order (7)
I believe the answer is:
sustain
'keep' is the definition.
(I know that keep can be written as sustain)
'aunt and sis in order' is the wordplay.
'and' means one lot of letters go next to another.
'in order' indicates an anagram (the letters in a new order).
'aunt'+'sis'='auntsis'
'auntsis' with letters rearranged gives 'SUSTAIN'.
(Other definitions for sustain that I've seen before include "Maintain" , "Withstand" , "Give food to" , "Undergo" , "Strengthen, support" .)