A fourth tequila drunk (plus the fifth in reserve also to be drunk) (8)
I believe the answer is:
quartile
'a fourth tequila drunk plus the fifth' is the definition.
I can't judge whether this defines the answer.
'reserve also to be drunk' is the wordplay.
'reserve also' becomes 'quare' (I can't explain this - if you can you should believe this answer much more).
'to' becomes 'til' (I've seen this in another clue).
'be drunk' means one lot of letters goes inside another (inserted letters are drunk in).
'quare' placed around 'til' is 'QUARTILE'.
'in' is the link.
Can you help me to learn more?
(Another definition for quartile that I've seen is " A fourth part of a distribution (stats)".)