H, sometimes (3)
I believe the answer is:
eta
'h sometimes' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for eta that I've seen before include "When you'd expect to see" , "when you can hope to receive it" , "Greek letter E; estimated time of arrival (abbr.)" , "Member of Greek set a" , "Seventh Greek letter" .)