The most common and most likely answer to this crossword clue is the 8 letter word LOVEBIRD
I've seen this crossword clue in The Daily Telegraph
L
O
V
E
B
I
R
D
Buddy explains!
The straight part
We know that "romantically demonstrative partner" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case romantically demonstrative partner = lovebird.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does LOVEBIRD mean?
LOVEBIRD noun- Any small parrot from one of the nine species within the genus Agapornis. Sometimes they are kept as cage birds and are noted for their affection towards each other.
- (usually in the plural) One of the members of an openly affectionate couple.