Frightful person drinking vermouth before five brought back dizziness (7)
I believe the answer is:
vertigo
'dizziness' is the definition.
(synonyms)
'frightful person drinking vermouth before five brought back' is the wordplay.
'frightful person' becomes 'ogre' (ogre is a type of monster).
'drinking' indicates putting letters inside (inserted letters are consumed or drunk).
'vermouth' becomes 'it' (short for 'Italian vermouth' as in 'gin and it').
'before five' becomes 'v' (Roman numeral. I am not sure about the 'before' bit.).
'brought back' shows that the letters should be reversed in order.
'ogre' going around 'it' is 'ogitre'.
'ogitre'+'v'='ogitrev'
'ogitrev' written backwards gives 'VERTIGO'.
(Other definitions for vertigo that I've seen before include "Sensation of dizziness brought on by heights" , "Sensation brought on by great heights" , "unbalanced feeling" , "Sensitivity to heights" , "Forty Thieves' password" .)