Measure litres, then stupidly drinking gallons (6)
I believe the answer is:
length
'measure' is the definition.
(length is a kind of measure)
'litres then stupidly drinking gallons' is the wordplay.
'litres' becomes 'l'.
'stupidly' is an anagram indicator.
'drinking' indicates putting letters inside.
'gallons' becomes 'g'.
'then' anagrammed gives 'enth'.
'enth' enclosing 'g' is 'ength'.
'l'+'ength'='LENGTH'
(Other definitions for length that I've seen before include "stretch" , "A brief swim" , "End-to-end distance" , "A bit of swimming" , "Extent - distance" .)