Very good marks, for example, in test (5)
I believe the answer is:
assay
'test' is the definition.
(I know that assay is a type of test)
'very good marks for example' is the wordplay.
'very good marks' becomes 'as' (I can't justify this - if you can you should give a lot more credence to this answer).
'for example' becomes 'say' ('say' can be used to introduce an example).
'as'+'say'='ASSAY'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for assay that I've seen before include "Analyse to find the properties" , "Trial" , "Test gold" , "Test ore for content" , "Test the purity of metals" .)