Wayward lassie ultimately into drink? (7)
I believe the answer is:
ladette
'wayward lassie ultimately into drink?' is the definition.
(boisterous, heavy drinking young woman)
'wayward lassie ultimately into drink?' is the wordplay.
'ultimately' suggests the final letters.
'into' is an insertion indicator.
'drink?' becomes 'latte' (I've seen this before).
The last letters of 'wayward lassie' are 'de'.
'de' going inside 'latte' is 'LADETTE'.
(Other definitions for ladette that I've seen before include "Rowdy youngster" , "Loutish female" , "Girl likely to drink lots" , "Young female behaving like a lout" , "Crude young woman" .)