A yell, an alarm signal and a cry of pain (6)
I believe the answer is:
bellow
'a yell' is the definition.
(bellow is a kind of yell)
'an alarm signal and a cry of pain' is the wordplay.
'an alarm signal' becomes 'bell' (I can't justify this - if you can you should believe this answer much more).
'and' says to put letters next to each other.
'a cry of pain' becomes 'ow' (exclamation of pain).
'bell'+'ow'='BELLOW'
Can you help me to learn more?
(Other definitions for bellow that I've seen before include "Roar, shout" , "Roar loudly" , "Loud deep sound as by bull" , "author" , "Roar like a bull" .)