Principal playing in glade (7)
I believe the answer is:
leading
'principal' is the definition.
(I've seen this before)
'playing in glade' is the wordplay.
'playing' is an anagram indicator.
'in'+'glade'='inglade'
'inglade' anagrammed gives 'LEADING'.
(Other definitions for leading that I've seen before include "In pole position" , "in van" , "Most important; ahead" , "'Showing the way, out in front (7)'" , "Female top of the bill" .)