Concerning a union, almost spoil the entire thing (7)
I believe the answer is:
marital
'concerning a union almost' is the definition.
I can't tell whether this definition defines the answer.
'spoil the entire thing' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'the entire thing' becomes 'ital' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'mar'+'ital'='MARITAL'
Can you help me to learn more?
(Other definitions for marital that I've seen before include "Of nuptials" , "to do with couple's relationship" , "to do with life together" , "Relating to wedded life" , "Maybe husband's" .)