Gunners line up during morning panic (5)
I believe the answer is:
alarm
'panic' is the definition.
(alarm can mean panic or fear)
'gunners line up during morning' is the wordplay.
'gunners' becomes 'RA' (Royal Artillery).
'line' becomes 'l' (used when specifying particular lines from a poem).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'during' is an insertion indicator.
'morning' becomes 'am' (as in 12:00 AM).
'ra'+'l'='ral'
'ral' reversed gives 'lar'.
'lar' put within 'am' is 'ALARM'.
(Other definitions for alarm that I've seen before include "Put the wind up" , "Deep concern" , "uneasiness" , "Consternation: kind of clock" , "Burglar deterrent" .)