Beer initially on verge of running out? Calamity (4)
I believe the answer is:
blow
'calamity' is the definition.
(thesaurus)
'beer initially on verge of running out?' is the wordplay.
'initially' suggests taking the first letters.
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'verge of running out?' becomes 'low' (I can't justify this - if you can you should give a lot more credence to this answer).
The initial letter of 'beer' is 'b'.
'b'+'low'='BLOW'
Can you help me to learn more?
(Other definitions for blow that I've seen before include "Clip" , "Hit - disappointment" , "Strong current of air" , "cocaine from US" , "Whistle - hit" .)