Like, for instance, the start of sheepdog trials (6)
I believe the answer is:
assays
'trials' is the definition.
'like for instance the start of sheepdog' is the wordplay.
'like' becomes 'as'.
'for instance' becomes 'say'.
'the start of' indicates taking the first letters.
The initial letter of 'sheepdog' is 's'.
'as'+'say'+'s'='ASSAYS'
(Other definitions for assays that I've seen before include "Determines the proporation of gold in a nugget" , "Does a test" , "Tests to determine metallic content" , "Tests ore for quality" , "Tests (a metal)" .)