King Lear's mad, holding heartless test to reveal snakes (8)
I believe the answer is:
rattlers
'snakes' is the definition.
(rattler is a kind of snake)
'king lear's mad holding heartless test' is the wordplay.
'king' becomes 'R' (abbreviation of Latin rex).
'mad' indicates an anagram.
'holding' means one lot of letters goes inside another.
'heartless' means to remove the middle letters.
'test' with its middle removed is 'tt'.
'lears' anagrammed gives 'alers'.
'alers' enclosing 'tt' is 'attlers'.
'r'+'attlers'='RATTLERS'
'to reveal' is the link.
(Another definition for rattlers that I've seen is " snakes".)