Number imprisoned by extremely vile Frenchman showing spite (5)
I believe the answer is:
venom
'spite' is the definition.
(synonyms)
'number imprisoned by extremely vile frenchman' is the wordplay.
'number' becomes 'no' (abbreviation).
'imprisoned by' is an insertion indicator.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'frenchman' becomes 'M' (abbreviation for Monsieur - like 'Mr').
'vile' with its middle removed is 've'.
've'+'m'='vem'
'no' going inside 'vem' is 'VENOM'.
'showing' is the link.
(Other definitions for venom that I've seen before include "Bitter malice" , "Spite: toxin" , "bitterness" , "Virulence" , "Poisonous fluid" .)