Threatening noise will increase in magnitude and length (5)
I believe the answer is:
growl
'threatening noise' is the definition.
The answer and definition can be both related to natural events as well as being singular nouns.
Maybe there's a link between them I don't understand?
'will increase in magnitude and length' is the wordplay.
I cannot quite understand how this works, but
'increase' could be 'grow' (growing is a kind of increasing) and 'grow' is found in the answer.
'length' could be 'l' (abbreviation in geometry) and 'l' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for growl that I've seen before include "Threatening dog's noise" , "bare teeth" , "(Make a) menacing sound" , "warn of attack" , "snap" .)