Quietly feeding dogs upset by simple tail trim (6)
I believe the answer is:
spruce
'trim' is the definition.
('spruce' can be a synonym of 'trim')
'quietly feeding dogs upset by simple tail' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'feeding' means one lot of letters goes inside another.
'dogs' becomes 'curs' (cur can mean a violent dog).
'upset' shows that the letters should be reversed in order.
'by simple tail' becomes 'e' (I can't justify this - if you can you should believe this answer much more).
'p' put into 'curs' is 'curps'.
'curps' reversed gives 'spruc'.
'spruc'+'e'='SPRUCE'
(Other definitions for spruce that I've seen before include "Smart in appearance like fir tree" , "Coniferous tree" , "thing for fairy to be on top of?" , "tree with needle-like leaves" , "Neat and smart like pine tree" .)