Quiet, reticent person has a change of heart (4)
I believe the answer is:
calm
'quiet' is the definition.
(I know that quiet can be written as calm)
'reticent person has a change of heart' is the wordplay.
'reticent person' becomes 'alm' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'has' says to put letters next to each other.
'a change' becomes 'ace' (ace is a kind of change).
'of heart' means to look at the middle letters (I've seen 'heart of' mean this).
The middle letter of 'ace' is 'c'.
'alm' after 'c' is 'CALM'.
(Other definitions for calm that I've seen before include "Still or peaceful" , "Unruffled, not excited" , "Serene" , "Peaceful and unexcited" , "Placid, unexcited" .)