Second husband boarding air-conditioned train (6)
I believe the answer is:
school
'train' is the definition.
(both can mean to teach or coach)
'second husband boarding air-conditioned' is the wordplay.
'second' becomes 's' (common abbreviation).
'husband' becomes 'h' (abbreviation).
'boarding' is an insertion indicator.
'air-conditioned' becomes 'cool' (similar in meaning).
'h' placed inside 'cool' is 'chool'.
's'+'chool'='SCHOOL'
(Other definitions for school that I've seen before include "Academy; group of fish" , "group of swimmers" , "Group of pupils, fish" , "See 7" , "Teach - fishes swimming" .)