Nothing in schedule for early morning caller (7)
I believe the answer is:
rooster
'early morning caller' is the definition.
(roosters call out in the morning)
'nothing in schedule' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'in' is an insertion indicator.
'schedule' becomes 'roster' (I've seen this before).
'o' put into 'roster' is 'ROOSTER'.
'for' is the link.
(Other definitions for rooster that I've seen before include "male with comb" , "Early-morning caller" , "Farmyard cock" , "So retro (anag.)" , "Chanticleer" .)