Baby finally, by daybreak, dropping head to show sign of tiredness (4)
I believe the answer is:
yawn
'sign of tiredness' is the definition.
(I've seen this before)
'baby finally by daybreak dropping head' is the wordplay.
'finally' says to take the final letters.
'by' says to put letters next to each other.
'daybreak' becomes 'dawn' (synonyms).
'dropping head' means to remove the first letter.
The final letter of 'baby' is 'y'.
'dawn' with its initial letter removed is 'awn'.
'y'+'awn'='YAWN'
'to show' acts as a link.
(Other definitions for yawn that I've seen before include "Gape in tiredness" , "Tedious thing" , "Show one is tired or bored" , "Boring event" , "Open mouth wide" .)