Sadly, Mabel and Ella are easily influenced (9)
I believe the answer is:
malleable
'easily influenced' is the definition.
(I've seen this before)
'sadly mabel and ella' is the wordplay.
'sadly' indicates anagramming the letters.
'and' is a charade indicator (letters next to each other) (I've seen this in other clues).
'mabel' after 'ella' is 'ellamabel'.
'ellamabel' is an anagram of 'MALLEABLE'.
'are' acts as a link.
(Other definitions for malleable that I've seen before include "Being adaptable" , "Compliant" , "Able to be hammered" , "Easily influenced" , "Able to be shaped" .)