In Omsk yearly: heavens! (3)
I believe the answer is:
sky
'heavens' is the definition.
(sky is a kind of heavens)
'in omsk yearly' is the wordplay.
'in' indicates the answer is hidden within the clue.
'SKY' is hidden within 'omsk yearly'.
(Other definitions for sky that I've seen before include "Hit high into the air" , "Strike up" , "-- Blue" , "Orbiting space-station" , "broadcaster" .)