One following second speaker (7)
I believe the answer is:
stalker
'one following' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'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 "Obsessive follower" , "One harassing celebrity" , "Man who follows and intimidates a woman" , "Someone following another, sneakily" , "Obsessive harasser" .)