Hearing this, we hear a hanky's required! (7)
I believe the answer is:
atishoo
'hanky's required' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.
'hearing this we hear a' is the wordplay.
I cannot quite see how this works, but
an anagram of 'this' is 'tish' which is within the answer.
'a' is within the answer.
The remaining letters 'oo' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for atishoo that I've seen before include "Sound of a sneeze" , "Sound of explosion" , "Word to be sneezed at" , "Sound made on expiration" , "Sneezing sound" .)