Top environmental scientist interrupts pioneer's exams (5)
I believe the answer is:
tests
'exams' is the definition.
('test' can be a synonym of 'exam')
'top environmental scientist interrupts pioneer's' is the wordplay.
'top' says to take the initial letters (in a down clue, the first letter is top).
'interrupts' means one lot of letters goes inside another.
'pioneer's' becomes 'tts' (I can't justify this - if you can you should give a lot more credence to this answer).
The first letters of 'environmental scientist' is 'es'.
'es' inserted into 'tts' is 'TESTS'.
Can you help me to learn more?
(Other definitions for tests that I've seen before include "Trials, exams" , "Tries out" , "Trials for cricketers" , "Examinations, trials" , "Trials -- tribulations" .)