Letter that he takes to heart (5)
I believe the answer is:
theta
'letter that he takes to heart' is the definition.
'theta' can be an answer for 'letter' (Greek letter). I am not certain of the remainder of the definition.
'letter that he takes to heart' is the wordplay.
'letter' indicates anagramming the letters (I've seen 'letters' mean this).
'he takes' becomes 'e' (I am not sure about this - if you are sure you should believe this answer much more).
'to heart' indicates putting letters inside.
'that' anagrammed gives 'thta'.
'thta' placed around 'e' is 'THETA'.
Can you help me to learn more?
(Other definitions for theta that I've seen before include "Eighth letter of Greek alphabet" , "letter from Athens" , "Make amends for" , "classic character?" , "Letter from Greece" .)