Intercept principal on holiday (4,3)
I believe the answer is:
head off
'intercept' is the definition.
(I've seen this before)
'principal on holiday' is the wordplay.
'principal' becomes 'head' ('head' can be a synonym of 'principal'**).
'on holiday' becomes 'off' (I've seen this before).
'head'+'off'='HEAD OFF'
(Other definitions for head off that I've seen before include "Turn away" , "Intercept - at the pass?" , "prevent" , "Stop" , "result of execution?" .)