Lots of girls in class using heads for reasoning (5)
I believe the answer is:
logic
'for reasoning' is the definition.
(I've seen this before)
'lots of girls in class using heads' is the wordplay.
'lots of' becomes 'c'.
'girls in class using' becomes 'logi' (I can't justify this - if you can you should give a lot more credence to this answer).
'heads' means one lot of letters go next to another (some letters go ahead of others).
'c' after 'logi' is 'LOGIC'.
Can you help me to learn more?
(Other definitions for logic that I've seen before include "Sound reasoning" , "Reasonable judgement" , "System of reasoning" , "Aristotelian theories" , "Chain of reasoning" .)