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