Indication to stop studied when speaking (3)
I believe the answer is:
red
'indication to stop' is the definition.
(red traffic light)
'studied when speaking' is the wordplay.
'studied' becomes 'read' (I've seen this before).
'when speaking' indicates a 'sounds like' (homophone) clue.
'read' is a homophone of 'RED'.
(Other definitions for red that I've seen before include "Colour for danger" , "Warning light colour" , "Carmine" , "Very angry, seeing this" , "River, its source in the US, flowing north to empty into L Winnipeg" .)