Emus are tricky to evaluate (7)
I believe the answer is:
measure
'evaluate' is the definition.
(I know that evaluate is a more specific form of evaluate)
'emus are tricky' is the wordplay.
'tricky' indicates anagramming the letters.
'emus'+'are'='emusare'
'emusare' anagrammed gives 'MEASURE'.
'to' acts as a link.
(Other definitions for measure that I've seen before include "Tape" , "Show something's dimensions" , "Quantify" , "Determine the size of" , "Evaluate; ruler" .)