Crossword Genius

Neatly replacing one with daughter, Merry (6)

Ross

I believe the answer is:

tiddly

'merry' is the definition.
(both can mean slightly drunk)

I cannot understand how the remainder of the clue works.

(Other definitions for tiddly that I've seen before include "drop of booze could make one this" , "A little drunk" , "Slightly drunk or very small" , "Diminutive - drunk" , "Tiny" .)

I've seen this clue in The Guardian.
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