Schedule disrupted by old male calling at first light (7)
I believe the answer is:
rooster
'male calling at first light' is the definition.
I can't judge whether this defines the answer.
'schedule disrupted by old' is the wordplay.
'schedule' becomes 'roster' (I've seen this before).
'disrupted by' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'roster' placed around 'o' is 'ROOSTER'.
Can you help me to learn more?
(Other definitions for rooster that I've seen before include "Male domestic fowl (US)" , "Early-morning caller" , "Morning caller" , "Adult male domestic fowl" , "Chanticleer" .)