Blunder in fear, losing head (5)
I believe the answer is:
error
'blunder' is the definition.
(thesaurus)
'fear losing head' is the wordplay.
'fear' becomes 'terror' (terror is a kind of fear).
'losing head' means to remove the first letter.
'terror' with its first letter removed is 'ERROR'.
'in' is the link.
(Other definitions for error that I've seen before include "Way of operating" , "heresy" , "Faulty judgment" , "Mistake or mishap" , "A mistake or inaccuracy" .)