Nothing separating boy and girl's principles (10)
I believe the answer is:
philosophy
'principles' is the definition.
(a set of principles or beliefs)
'nothing separating boy and girl's' is the wordplay.
I cannot quite understand how this works, but
'nothing' could be 'o' (looks like zero - 0) and 'o' is found in the answer.
'boy' could be 'phil' and 'phil' is found within the answer.
The remaining letters 'sophy' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for philosophy that I've seen before include "First P of PPE" , "School of thought" , "Outlook" , "Eg, Stoicism" , "Pursuit of truth and understanding" .)