Eavesdrops on last campfire song (4)
I believe the answer is:
taps
I believe this clue is a double definition.
'eavesdrops' is the first definition.
(tapping is a kind of eavesdroping)
'last campfire song' is the second definition.
(bugle call ending American scouts' campfires)
'on' is the link.
(Other definitions for taps that I've seen before include "Bugs (wire)" , "Exploits" , "Uses listening devices" , "Strikes lightly; eavesdropping devices" , "pierces" .)