Second speaker – one engaged in stealthy pursuit (7)
I believe the answer is:
stalker
'one engaged in stealthy pursuit' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'second speaker' is the wordplay.
'second' becomes 's' (common abbreviation).
'speaker' becomes 'talker' (synonyms).
's'+'talker'='STALKER'
Can you help me to learn more?
(Other definitions for stalker that I've seen before include "Stealthy pursuer" , "Person who follows another about as if hunting prey" , "Someone following another, sneakily" , "Man who follows and intimidates a woman" , "One who pursues prey stealthily" .)