Form again on playing-field – old school head's gone missing (12)
I believe the answer is:
reconstitute
'form again' is the definition.
('constitute' can be a synonym for 'form' so perhaps 'reconstitute' is OK for 'form again')
'playing-field old school head's gone missing' is the wordplay.
I cannot really see how this works, but
'playing' could be 'on' (on can mean playing or performing) and 'on' is found in the answer.
'field' could be 'rec' (short for recreation ground) and 'rec' is found within the answer.
'school' could be 's' and 's' is found within the answer.
This explanation may well be incorrect...
'on' acts as a link.
Can you help me to learn more?
(Other definitions for reconstitute that I've seen before include "Restore" , "Reform" , "Reassemble from parts" , "Restructure" .)