In full, it represents a measure of liquid (5)
I believe the answer is:
litre
'a measure of liquid' is the definition.
(I've seen this before)
'in full it represents' is the wordplay.
'in' indicates the answer is hidden within the clue.
'LITRE' is hidden within 'full it represents'.
(Other definitions for litre that I've seen before include "Unit of liquid measure" , "One cubic decimetre" , "Nearly two pints" , "One thousand cubic centimetres" , "quantity of drink" .)