Sweetheart or duck, in crossword terms? (8)
I believe the answer is:
lovebird
'sweetheart' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'duck in crossword terms?' is the wordplay.
'duck' becomes 'love' (duck and love mean zero in cricket and tennis).
'in crossword terms?' becomes 'bird' (I am not sure about this - if you are sure you should believe this answer much more).
'love'+'bird'='LOVEBIRD'
'or' is the link.
Can you help me to learn more?
(Other definitions for lovebird that I've seen before include "Small (affectionate) parrot" , "Canoodler" , "Small African parrot" , "Small green parrot" .)