Just behind front of fire alarm (6)
I believe the answer is:
fright
'alarm' is the definition.
(both can mean fear or worry)
'just behind front of fire' is the wordplay.
'just' becomes 'right' (associated in meaning).
'behind' says to put letters next to each other.
'front of' suggests taking the first letters (the letter at the front of the word).
The initial letter of 'fire' is 'f'.
'right' after 'f' is 'FRIGHT'.
(Other definitions for fright that I've seen before include "Panic" , "Start" , "Sudden exposure to fear" , "Shock, sudden alarm" , "Alarming event" .)