Portend odds of bloody end to blockade (4)
I believe the answer is:
bode
'portend' is the definition.
(synonyms)
'odds of bloody end to blockade' is the wordplay.
'odds of' means one should take alternating letters (the letters in odd-numbered positions are used).
'end to' indicates one should take the final letters.
The final letter of 'blockade' is 'e'.
The alternate letters of 'bloody' are 'bod'.
'bod'+'e'='BODE'
(Other definitions for bode that I've seen before include "Augur, foreshadow" , "Portend, foretell" , "Presage" , "predict" , "Waited" .)