Pleasure has to be taking in quiet trees (9)
I believe the answer is:
happiness
'pleasure' is the definition.
(I know that pleasure can be written as happiness)
'has to be taking in quiet trees' is the wordplay.
'to be taking in' is an insertion indicator.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'trees' becomes 'pines' (pine is a kind of tree).
'p'+'pines'='ppines'
'has' going around 'ppines' is 'HAPPINESS'.
(Other definitions for happiness that I've seen before include "Felicity" , "Joy, well-being," , "State of well-being, contentment" , "Contentment, joy" , "Enjoyment" .)