To appear in constant controlling device (9)
I believe the answer is:
regulator
'controlling device' is the definition.
(I've seen this before)
'to appear in constant' is the wordplay.
'appear in' means one lot of letters goes inside another.
'constant' becomes 'regular' (I can't justify this - if you can you should believe this answer much more).
'to' placed within 'regular' is 'REGULATOR'.
Can you help me to learn more?
(Other definitions for regulator that I've seen before include "Governor" , "Controller" , "Of body" , "Ofsted perhaps" , "part of the watch" .)