Threaten to invade garden plot that was in flower (7)
I believe the answer is:
bloomed
'was in flower' is the definition.
I can't judge whether this defines the answer.
'threaten to invade garden plot' is the wordplay.
'threaten' becomes 'loom' (looming is a kind of threatening).
'to invade' is an insertion indicator (inserted letters invade the word).
'garden plot' becomes 'bed' (I've seen this before).
'loom' placed into 'bed' is 'BLOOMED'.
'that' is the link.
Can you help me to learn more?
(Other definitions for bloomed that I've seen before include "Came into flower" , "developed" .)