Give instruction: sun acceptable when hot internally (6)
I believe the answer is:
school
'give instruction' is the definition.
(school can mean to teach or instruct)
'sun acceptable when hot internally' is the wordplay.
'sun' becomes 'S' (abbreviation).
'acceptable' becomes 'cool' ('that's cool' can mean that's fine or acceptable).
'when hot' becomes 'h' (abbreviation eg on taps. I am not sure about the 'when' bit.).
'internally' indicates putting letters inside.
'cool' going around 'h' is 'chool'.
's'+'chool'='SCHOOL'
(Other definitions for school that I've seen before include "shoal of fish" , "some bass?" , "Group of pupils, fish" , "Group of sea creatures" , "company that's all at sea" .)