Make an effort with conservative, withholding nothing (3)
I believe the answer is:
try
'make an effort' is the definition.
(to make an effort is to sincerely try)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for try that I've seen before include "crack" , "A rugby score, result of effort" , "Gorse" , "Attempt a score" , "Bid" .)