Leaning forward playing the cello? (6)
I believe the answer is:
bowing
This clue is a double definition.
'leaning forward' is the first definition.
(to bow is to lean forward)
'playing the cello?' is the second definition.
(a cello is played with a bow)
(Other definitions for bowing that I've seen before include "musical technique" , "deforming with age" , "Bending" , "Acknowledging applause" , "Perhaps playing Strad" .)