Help mug to be heard (7)
I believe the answer is:
succour
'help' is the definition.
(I know that succour is a type of help)
'mug to be heard' is the wordplay.
'mug' becomes 'sucker' (synonyms).
'to be heard' shows a homophone (sound like).
'sucker' sounds like 'SUCCOUR'.
(Other definitions for succour that I've seen before include "Help given in time of need" , "Encourage" , "helping hand" , "Help, relief" , "Give assistance in time of difficulty" .)