An HQ heard someone expressing agreement (8)
I believe the answer is:
assenter
'an hq heard someone' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'agreement' is the wordplay.
I cannot really see how this works, but
'agreement' could be 'assent' (assent is a kind of agreement) and 'assent' is located in the answer.
The remaining letters 'er' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'expressing' acts as a link.
Can you help me to learn more?
(Other definitions for assenter that I've seen before include "One who agrees" , "he's quite agreeable" .)