A letter marking the spot, we hear, for extension (6)
I believe the answer is:
annexe
'extension' is the definition.
(I know that annexe is a class)
'a letter marking the spot we hear' is the wordplay.
I cannot quite see how this works, but
'a' could be 'an' and 'an' is present in the answer.
'letter' could be 'ex' (X) and 'ex' is found within the answer.
The remaining letters 'ne' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'for' is the link.
Can you help me to learn more?
(Other definitions for annexe that I've seen before include "Extension, supplement" , "extra space" , "Building attached to a larger one" , "Conservatory, e.g." , "Extra accommodation block" .)