Stop — you're heard last (6)
I believe the answer is:
endure
'heard last' is the definition.
'endure' can be an answer for 'last' (I've seen this before). I am not sure about the 'heard' bit.
'stop you're' is the wordplay.
'stop' becomes 'end' (synonyms).
'you're' becomes 'ure' (I can't justify this - if you can you should give a lot more credence to this answer).
'end'+'ure'='ENDURE'
(Other definitions for endure that I've seen before include "Bear, suffer" , "Persist over time" , "Tolerate, put up with" , "Suffer or undergo" , "Stand" .)