Enforce a general embargo, mostly lacking impact (5)
I believe the answer is:
banal
'enforce a general' is the definition.
I can't judge whether this defines the answer.
'embargo mostly lacking impact' is the wordplay.
'embargo' becomes 'ban' (I've seen this before).
'mostly lacking impact' becomes 'al' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ban'+'al'='BANAL'
Can you help me to learn more?
(Other definitions for banal that I've seen before include "Obvious and dull" , "Boring or trivial" , "Uninteresting, commonplace" , "'Dull, commonplace (5)'" , "Obvious and boring" .)