Trial of brute strength (4)
I believe the answer is:
test
'trial' is the definition.
(synonyms)
'of brute strength' is the wordplay.
'of' indicates a hidden word.
'TEST' can be found hidden inside 'brute strength'.
(Other definitions for test that I've seen before include "Cricket match; try" , "big game" , "Dip one's toe in" , "water flowing through Hampshire" , "Trial; cricket match" .)