What about plan received by sensitive signalling system? (9)
I believe the answer is:
semaphore
'signalling system?' is the definition.
(I've seen this before)
'what about plan received by sensitive' is the wordplay.
'what' becomes 'eh' (I've seen this in other clues).
'about' indicates putting letters inside.
'plan' becomes 'map' (I've seen this before).
'received by' means one lot of letters goes inside another.
'sensitive' becomes 'sore' ('sore' can be a synonym of 'sensitive').
'eh' going around 'map' is 'emaph'.
'emaph' going into 'sore' is 'SEMAPHORE'.
(Other definitions for semaphore that I've seen before include "Old flag signalling system" , "Signalling system using flags" , "Messaging by flags" , "Strangely, home as per a system of communication" , "Old communications system" .)