Actual component of fire alarm (4)
I believe the answer is:
real
'actual' is the definition.
(I know that actual can be written as real)
'component of fire alarm' is the wordplay.
'component of' indicates a hidden word (I've seen this in other clues).
'REAL' is hidden in the letters of 'fire alarm'.
(Other definitions for real that I've seen before include "Type of ale" , "Actual and tangible" , "Actual; coin" , "Pukka" , "Unaffected" .)