Temperature having dropped, fear slip (5)
I believe the answer is:
error
'slip' is the definition.
(slip can mean an error or mistake)
'temperature having dropped fear' is the wordplay.
'temperature' becomes 't' (abbreviation).
'having dropped' is a deletion indicator.
'fear' becomes 'terror' (terror is a kind of fear).
'terror' with 't' taken out is 'ERROR'.
(Other definitions for error that I've seen before include "Miscalculation" , "Mistake from terrorist" , "heresy" , "Way of operating" , "Slip, mistake" .)