Think seriously about the chicks (5)
I believe the answer is:
brood
This clue might be a double definition.
'think seriously' is the first definition.
'brood' can be an answer for 'think' (thesaurus). I am not certain of the 'seriously' bit.
'the chicks' is the second definition.
(number of young birds hatched at the same time)
'about' acts as a link.
Can you help me to learn more?
(Other definitions for brood that I've seen before include "Worry over" , "Meditate moodily" , "Offspring" , "Sulk; incubate" , "Ponder - family" .)