Love, marry, it's an obligation (4)
I believe the answer is:
owed
'it's an obligation' is the definition.
I can't tell whether this defines the answer.
'love marry' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'marry' becomes 'we'd' (synonyms).
'o'+'wed'='OWED'
Can you help me to learn more?
(Other definitions for owed that I've seen before include "Outstanding" , "Was due to pay" , "(Sum) due for payment" , "Had a duty to pay" , "Was indebted" .)