Threaten to take head off flower (4)
I believe the answer is:
loom
'threaten' is the definition.
(looming is a kind of threatening)
'head off flower' is the wordplay.
'head off' means to remove the first letter.
'flower' becomes 'bloom' (synonyms).
'bloom' with its initial letter removed is 'LOOM'.
'to take' acts as a link.
(Other definitions for loom that I've seen before include "Impend; weaving machine" , "Textile machine; come into sight" , "Weaving frame - seem ominously close" , "Mill machinery" , "Appear dimly; be imminent" .)