Measure litres then stupidly put in gallons (6)
I believe the answer is:
length
'measure' is the definition.
(length is a kind of measure)
'litres then stupidly put in gallons' is the wordplay.
'litres' becomes 'L' (abbreviation).
'stupidly' indicates anagramming the letters (a stupid ordering of the letters).
'put in' indicates putting letters inside (instruction to solver).
'gallons' becomes 'g' (abbreviation).
'then' is an anagram of 'enth'.
'enth' going around 'g' is 'ength'.
'l'+'ength'='LENGTH'
(Other definitions for length that I've seen before include "Duration or distance" , "End-to-end distance" , "Something's largest dimension" , "Measure (to which one should bowl?)" , "A brief swim" .)