Set out to forestall principal on vacation (4,3)
I believe the answer is:
head off
'forestall principal on vacation' is the definition.
'head off' can be an answer for 'forestall' (heading off is a kind of forestalling). I'm unsure of the 'principal on vacation' bit.
'set out' is the wordplay.
'set' becomes 'head' (I am not sure about this - if you are sure you should believe this answer much more).
'out' becomes 'off' (I've seen this before).
'head'+'off'='HEAD OFF'
'to' is the link.
Can you help me to learn more?
(Other definitions for head off that I've seen before include "Turn away" , "Avert; start (in a direction)" , "go" , "Escape" , "prevent" .)