Influence two females echo in performance (6)
I believe the answer is:
affect
'influence' is the definition.
(I've seen this before)
'two females echo in performance' is the wordplay.
'two females' becomes 'ff' (an abbreviation for female written twice).
'echo' becomes 'e' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' indicates putting letters inside.
'performance' becomes 'act' (act is a kind of performance).
'ff'+'e'='ffe'
'ffe' placed within 'act' is 'AFFECT'.
(Other definitions for affect that I've seen before include "It always has a cause" , "attack" , "Touch (feelings); pretend (to feel)" , "Arouse sadness or sympathy like tear-jerker" , "Make a difference to" .)