Leaving in the middle of one's hair (7)
I believe the answer is:
parting
'leaving' is the definition.
(parting is a kind of leaving)
'in the middle of one's hair' is the wordplay.
I cannot quite see how this works, but
'in' is present in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
'one' could be 'a' (a thing is one thing) and 'a' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for parting that I've seen before include "splitting up" , "Separating" , "Leaving or dividing" , "Separation" , "Aggressive remarks on leaving (from a 20?)" .)