Try blocking extremely serious cuts (6)
I believe the answer is:
shears
'cuts' is the definition.
(shearing is a kind of cutting)
'try blocking extremely serious' is the wordplay.
'try' becomes 'hear' (as in to try someone in court or hear their case).
'blocking' indicates putting letters inside (to block can mean to plug a gap in something).
'extremely' suggests removing the centre (letters on the extremes of the word are used).
'serious' with its centre removed is 'ss'.
'hear' inserted within 'ss' is 'SHEARS'.
(Other definitions for shears that I've seen before include "cutter" , "Removes fleece" , "Big scissors" , "Large scissors for clipping sheep" , "something for lifting loads" .)