A letter is all the thanks you get (5)
I believe the answer is:
theta
'a letter' is the definition.
(Greek letter)
'all the thanks you get' is the wordplay.
I cannot really see how this works, but
an anagram of 'the' is 'het' which is located in the answer.
'thanks' could be 'ta' (UK informal version of 'thank you') and 'ta' is found in the answer.
No letters remain.
This explanation may well be incorrect...
'is' acts as a link.
Can you help me to learn more?
(Other definitions for theta that I've seen before include "letter from Athens" , "Make amends for" , "Greek character" , "Character from overseas" , "Greek letter used in trigonometry" .)