A warning of some blow to come (5,6)
I believe the answer is:
storm signal
'a warning' is the definition.
(I've seen this before)
'some blow to come' is the wordplay.
I cannot quite understand how this works, but
'some' could be 'a' and 'a' is found in the answer.
'blow' could be 'storm' (storming is a kind of blowing) and 'storm' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'of' is the link.
Can you help me to learn more?
(Another definition for storm signal that I've seen is " It's a warning of bad weather".)