Panic, losing head, making a mistake (5)
I believe the answer is:
error
'a mistake' is the definition.
(I know that mistake can be written as error)
'panic losing head' is the wordplay.
'panic' becomes 'terror' (synonyms).
'losing head' means to remove the first letter.
'terror' with its first letter taken away is 'ERROR'.
'making' is the link.
(Other definitions for error that I've seen before include "Slip, mistake" , "Faux pas" , "Mistake or mishap" , "1 down" , "Howler" .)