The way to conceal a switch (4)
I believe the answer is:
road
'the way' is the definition.
(road is a kind of way)
'to conceal a switch' is the wordplay.
'to conceal' is an insertion indicator.
'switch' becomes 'rod' (I've seen this in another clue).
'a' put inside 'rod' is 'ROAD'.
(Other definitions for road that I've seen before include "motorway?" , "Mine passage; way" , "See 2" , "Abbey -, Beatles album" , "Way to achieving a particular outcome" .)