He yawns uncontrollably: should I stop? (3,4)
I believe the answer is:
Here is my best explanation:
'should i stop?' is the definition.
(said when pouring a drink)
'he yawns uncontrollably' is the subsidiary indication.
'uncontrollably' is an anagram indicator (I've seen this in other clues).
'heyawns' with letters rearranged gives 'saywhen'.
(Other definitions for say when that I've seen before include "Tell me when to stop pouring", "Words from the pourer?", "Pourer's words" and "Request to be told to stop pouring drink".)