Actual component of fire alarm (4)
I believe the answer is:
real
'actual' is the definition.
(similar in meaning)
'component of fire alarm' is the wordplay.
'component of' indicates the answer is hidden within the clue (I've seen this in other clues).
'REAL' is hidden within 'fire alarm'.
(Other definitions for real that I've seen before include "Type of ale" , "Actual and tangible" , "Actual; coin" , "Pukka" , "Unaffected" .)