After vacation, enliven school board (7)
I believe the answer is:
entrain
'board' is the definition.
(entraining is a kind of boarding)
'after vacation enliven school' is the wordplay.
'after vacation' suggests removing the centre (the inside letters must vacate).
'school' becomes 'train' (training is a kind of schooling).
'enliven' with its centre taken out is 'en'.
'en'+'train'='ENTRAIN'
(Other definitions for entrain that I've seen before include "Pop on the coach" , "Bring about as consequence" , "Put on board at the station" , "Go on board at the station" , "Board a rail service" .)