Principal stable boy out East (4)
I believe the answer is:
lead
'principal' is the definition.
(I know that principal can be written as lead)
'stable boy out east' is the wordplay.
'stable boy' becomes 'lad' (informal term for boy. I am not sure about the 'stable' bit.).
'out' means one lot of letters goes inside another.
'east' becomes 'e' (abbreviation).
'lad' going around 'e' is 'LEAD'.
Can you help me to learn more?
(Other definitions for lead that I've seen before include "Live" , "Take the van with heavy metal" , "Guide; precede" , "in first place" , "Chief performer" .)