Dave heartlessly called exhausted leader to disturb (7)
I believe the answer is:
derange
'to disturb' is the definition.
(deranging is a kind of disturbing)
'dave heartlessly called exhausted leader' is the wordplay.
'heartlessly' means to remove the middle letters.
'called' becomes 'rang' (called on the phone).
'leader' indicates taking the first letters.
'dave' with its centre removed is 'de'.
The initial letter of 'exhausted' is 'e'.
'de'+'rang'+'e'='DERANGE'
(Other definitions for derange that I've seen before include "Make insane" , "Remove from order" , "Craze" , "Make mad" , "Unhinge" .)