Romeo into booze gives smile (4)
I believe the answer is:
grin
'smile' is the definition.
(I know that grin is a more specific form of the action smile)
'romeo into booze' is the wordplay.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'into' means one lot of letters goes inside another.
'booze' becomes 'gin' (gin is a kind of booze).
'r' placed within 'gin' is 'GRIN'.
'gives' acts as a link.
(Other definitions for grin that I've seen before include "Beam" , "Cheesy smile?" , "Smile - all that could be seen of the vanishing Cheshire Cat" , "show ivories happily with it" , "Happy expression" .)