Heard an opponent coming (7)
I believe the answer is:
arrival
'coming' is the definition.
(I know that coming can be written as arrival)
'heard an opponent' is the wordplay.
'heard' indicates a 'sounds like' (homophone) clue.
'an' becomes 'a'.
'opponent' becomes 'rival' (a rival is an opponent or enemy).
'a'+'rival'='arival'
'arival' is a homophone of 'ARRIVAL'.
(Other definitions for arrival that I've seen before include "Appearance (at a place)" , "Act of coming" , "Someone who's just come" , "Advent" , "Appearance, entrance" .)