Student tries out measures (6)
I believe the answer is:
litres
'measures' is the definition.
(litre is a kind of measure)
'student tries out' is the wordplay.
'student' becomes 'l' (as in L-plates for learner drivers).
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'tries' with letters rearranged gives 'itres'.
'l'+'itres'='LITRES'
(Other definitions for litres that I've seen before include "Cubic capacities" , "Metrical units of capacity" , "Liquid measures" , "Let Sir get the liquid amounts" , "Amounts of water" .)