Writer is tense after test (8)
I believe the answer is:
essayist
'writer' is the definition.
(essayist is a kind of writer)
'is tense after test' is the wordplay.
'tense' becomes 't' (abbreviation used in many dictionaries).
'after' means one lot of letters go next to another.
'test' becomes 'essay' ('essay' can be a synonym of 'test'**).
'is'+'t'='ist'
'ist' put after 'essay' is 'ESSAYIST'.
(Other definitions for essayist that I've seen before include "kind of writer" , "I write" , "Writer of short compositions" , "Writer (e.g. Lamb)" , "Writer of shorter pieces" .)