Volunteers to take the above letter (5)
I believe the answer is:
theta
'letter' is the definition.
(Greek letter)
'volunteers to take the above' is the wordplay.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'to take' becomes 'the' (I can't explain this - if you can you should believe this answer much more).
'the above' says to put letters next to each other (in a down clue, some letters go above others).
'ta' put after 'the' is 'THETA'.
Can you help me to learn more?
(Other definitions for theta that I've seen before include "character in Marathon" , "Make amends for" , "Foreign character" , "Letter from Greece" , "letter from Argos?" .)