Write down: 'Desist from wrapping snooker rod round mate's head' (8)
I believe the answer is:
document
'write down' is the definition.
(to document something is to write it down)
'desist from wrapping snooker rod round mate's head' is the wordplay.
'desist from' becomes 'don't' (don't do that).
'wrapping' means one lot of letters goes inside another.
'snooker rod' becomes 'cue' (I've seen this before).
'round' is an insertion indicator.
'head' says to take the initial letters.
The initial letter of 'mates' is 'm'.
'cue' enclosing 'm' is 'cume'.
'dont' going around 'cume' is 'DOCUMENT'.
(Other definitions for document that I've seen before include "Official paper" , "Piece of written matter" , "Paper, form" , "Don cut me (anag)" , "Written statement" .)