The lion is restless: call this urgently (7)
I believe the answer is:
hotline
'call this urgently' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'the lion is restless' is the wordplay.
'is restless' indicates anagramming the letters (letters not resting).
'the'+'lion'='thelion'
'thelion' is an anagram of 'HOTLINE'.
Can you help me to learn more?
(Other definitions for hotline that I've seen before include "Special phone link" , "Direct phone access (for presidents?)" , "Former direct phone line between the White House and the Kremlin" , "linked Washington to the Kremlin" , "Important number to call" .)