Was obliged to love and marry (4)
I believe the answer is:
owed
'was obliged to' is the definition.
(owed something to someone)
'love and marry' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'and' means one lot of letters go next to another.
'marry' becomes 'wed' (synonyms).
'o'+'wed'='OWED'
(Other definitions for owed that I've seen before include "Outstanding" , "to be paid" , "Was obliged to pay" , "Was indebted" , "Was due to pay" .)