Fill a glass (4)
I believe the answer is:
pour
'fill a glass' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for pour that I've seen before include "Cause to flow or rain heavily" , "16 [CATS AND DOGS]" , "Pelt - decant" , "Cause to flow as from pot to cup" , "Give out liquid" .)