Specimen mixed with elm sap (6)
I believe the answer is:
sample
'specimen' is the definition.
(I know that specimen can be written as sample)
'mixed with elm sap' is the wordplay.
'mixed' is an anagram indicator.
'with' is a charade indicator (letters next to each other).
'elm'+'sap' is 'elmsap'.
'elmsap' anagrammed gives 'SAMPLE'.
(Other definitions for sample that I've seen before include "Small part representing the whole" , "Drop of" , "Specimen, try out" , "Small part representative of the whole" , "Small part intended to represent the whole" .)