Nieces excited about first bit of chemical knowledge (7)
I believe the answer is:
science
'knowledge' is the definition.
(I have seen 'Systematic knowledge ' mean 'science' so perhaps 'knowledge' could also mean 'science')
'nieces excited about first bit of chemical' is the wordplay.
'excited' is an anagram indicator.
'about' indicates putting letters inside.
'first bit of' says to take the initial letters.
The initial letter of 'chemical' is 'c'.
'nieces' is an anagram of 'sience'.
'sience' going around 'c' is 'SCIENCE'.
(Other definitions for science that I've seen before include "Organised (search for) knowledge" , "something learnt at school" , "Subject such as biology, physics, etc" , "Christian --; -- fiction" , "Critically tested knowledge" .)