Southern fan guards principal in Harrow train (6)
I believe the answer is:
school
'train' is the definition.
(both can mean to teach or coach)
'southern fan guards principal in harrow' is the wordplay.
'southern' becomes 's' (abbreviation).
'fan' becomes 'cool' (fan can mean to keep cool).
'guards' is an insertion indicator (inserted letters are guarded).
'principal in harrow' becomes 'h' (I can't justify this - if you can you should believe this answer much more).
'cool' going around 'h' is 'chool'.
's'+'chool'='SCHOOL'
Can you help me to learn more?
(Other definitions for school that I've seen before include "Teaching institution" , "eg Harrow" , "Where 2 takes place" , "Centre of learning" , "Educational establishment" .)