Clue: Negative duo that won't bond on record is entering retreat (4,4)
Answer: LONE PAIR
Explanation: Definition: Negative duo that won't bond
 [ON + EP(abbrev. for an "extended play record", a musical recording with more tracks than a single) ] **contained in**(is entering) LAIR(an animal's den/a retreat).
Defn: In chemistry, a …/a pair of (negatively charged) electrons in an atom in a molecule that are not shared by other atoms in that molecule, ie. are not in a chemical bond.