Husband boarding small trendy train (6)
I believe the answer is:
school
'train' is the definition.
(both can mean to teach or coach)
'husband boarding small trendy' is the wordplay.
'husband' becomes 'h' (abbreviation).
'boarding' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'trendy' becomes 'cool' (I can't explain this - if you can you should give a lot more credence to this answer).
's'+'cool'='scool'
'h' inserted within 'scool' is 'SCHOOL'.
Can you help me to learn more?
(Other definitions for school that I've seen before include "Group of pupils, fish" , "Centre of learning" , "shoal of fish" , "Group of porpoises - place of education" , "Educate; group" .)