Clue: Frightful person drinking vermouth before five brought back dizziness (7)
Answer: VERTIGO
Explanation: Definition: dizziness
 **Reversal of**(… brought back) [ OGRE(a frightful person/a monster) **containing**(drinking) IT(informal term for Italian vermouth) **plus**(before) V(Roman numeral for "five") ].
Defn: …/the sensation of being off-balance.