Tom's up for the test (3)
I believe the answer is:
mot
'the test' is the definition.
'tom's up' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'tom' written backwards gives 'MOT'.
'for' is the link.
(Other definitions for mot that I've seen before include "Word in France, bon perhaps" , "Witty saying" , "Clever remark" , "Irish girlfriend" , "type of test" .)