Keen on popular test opener at first! (4)
I believe the answer is:
into
'keen on' is the definition.
(I've seen this in another clue)
'popular test opener at first' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'at first' indicates taking the first letters.
The initial letters of 'test opener' is 'to'.
'in'+'to'='INTO'
(Other definitions for into that I've seen before include "Teachers' trade union" , "To a position within" , "Two -- six is three" , "Expression of division" , "Knowledgeable about" .)