Note feature of act (6,1)
I believe the answer is:
middle c
This clue is a double definition.
'note' is the first definition.
(middle C is a musical note)
'feature of act' is the second definition.
('aCt' has a C in the middle)
(Other definitions for middle c that I've seen before include "Note near piano centre" , "Note whose key is (almost) central on a piano keyboard" , "Tone" , "It's almost at the centre of a piano keyboard" , "what Jacob has?" .)