Rolling preserves verges of couch grass (6)
I believe the answer is:
snitch
'grass' is the definition.
(both mean to inform on or betray)
'rolling preserves verges of couch' is the wordplay.
'rolling' shows that the letters should be reversed in order.
'preserves' becomes 'tins' (tinning is a kind of preserving).
'verges of couch' becomes 'ch' (I can't justify this - if you can you should give a lot more credence to this answer).
'tins' in reverse letter order is 'snit'.
'snit'+'ch'='SNITCH'
Can you help me to learn more?
(Other definitions for snitch that I've seen before include "Grass" , "Whistleblower (informal)" , "I'm telling!" , "Inform on someone in slangy way" , "Telltale (sl.)" .)