The most common and most likely answer to this crossword clue is the 6 letter word HUGGER
I've seen this crossword clue in The Guardian
H
U
G
G
E
R
More clues leading to the result HUGGER
Buddy explains!

The straight part
We know that "Secret attacker/affectionate partner" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case secret attacker/affectionate partner = hugger.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 HUGGER mean?
HUGGER noun- One who hugs or embraces.
- To conceal; to lurk in ambush.