Get the better of fight knocking head off borstal boy (7)
I believe the answer is:
outplay
'get the better of' is the definition.
(I've seen this before)
'fight knocking head off borstal boy' is the wordplay.
'fight' becomes 'bout' ('bout' can be a synonym of 'fight').
'knocking head off' means to remove the first letter.
'borstal boy' becomes 'play' (I can't explain this - if you can you should give a lot more credence to this answer).
'bout' with its initial letter removed is 'out'.
'out'+'play'='OUTPLAY'
Can you help me to learn more?
(Other definitions for outplay that I've seen before include "Act better" , "Excel at games" , "Perform better than" , "Defeat (sport)" , "Get the better of" .)