What could prevent snooker player extending break? (4)
I believe the answer is:
rest
This clue is a double definition.
'what could prevent snooker player extending' is the first definition.
(a snooker player uses a rest to avoid stretching)
'break?' is the second definition.
(a rest is taking a break)
(Other definitions for rest that I've seen before include "Remainder; cease work" , "Balance" , "Remainder; be inactive" , "Surplus" , "lean" .)