Exercise books on cutting good rock (6)
I believe the answer is:
pebble
'good rock' is the definition.
'pebble' can be an answer for 'rock' (pebble is a kind of rock). I am not sure about the 'good' bit.
'exercise books on cutting' is the wordplay.
'exercise' becomes 'pe' (Physical Education).
'books' becomes 'bb' (two abbreviations for book).
'on' becomes 'leg' (cricket term, fielding positions).
'cutting' means to remove the last letter.
'leg' with its final letter removed is 'le'.
'pe'+'bb'+'le'='PEBBLE'
(Other definitions for pebble that I've seen before include "'Small, rounded stone (6)'" , "Smooth little stone" , "Little rock" , "Stone on beach" , "There are many on a stony beach" .)