Weakling losing head in terror (3)
I believe the answer is:
imp
'terror' is the definition.
'weakling losing head' is the wordplay.
'weakling' becomes 'wimp' (wimp is a kind of weakling).
'losing head' means to remove the first letter.
'wimp' with its initial letter removed is 'IMP'.
'in' acts as a link.
(Other definitions for imp that I've seen before include "Small mischievous spirit" , "Elf" , "Emperor" , "A small mischievous devil" , "A mischievous little devil" .)