Not just a little sip

The answer to this crossword question is “GULP“. The clue “Not just a little sip is BELT” indicates that the answer must be a word for a larger amount of liquid consumed. The word “Belt” suggests the action of swallowing something quickly and decisively, which matches the definition of “Gulp“. So, the word GULP refers to a larger amount of liquid being ingested quickly in one swallow, unlike a small sip.