Expand length and sound menacing (5)
I believe the answer is:
growl
'and sound menacing' is the definition.
I can't tell whether this definition defines the answer.
'expand length' is the wordplay.
'expand' becomes 'grow' (to grow is to expand in size).
'length' becomes 'l' (abbreviation in geometry).
'grow'+'l'='GROWL'
Can you help me to learn more?
(Other definitions for growl that I've seen before include "Hostile murmur" , "Sound of hostile animal" , "Threatening dog's noise" , "threatening sound" , "Emit low rumbling sounds" .)