Clue: Ring a bell, possibly, to keep snitch regularly alert (2,3,4)
Answer: ON THE BALL
Explanation: Ring a bell, possibly, to keep snitch regularly alert (2,3,4)

**O** (ring) +***(a bell)** [anag;*possibly*] *to keep* (s)**N**(i)**T**(c)**H **[regularly], so **O(NTH)EBALL